this was all that remained, the next day, of Cassidy's birthday cake - and thank goodness, because this thing was dense and rich. I'm long overdue for posting recipes for this "Mounds Bar" cake (chocolate and coconut), but it was so good that I wanted to share everything, along with my notes. This is a chocolate cake (made with a bit of coconut oil for subtle coconut flavor), with a thick coconut filling (modeled on a recipe for homemade mounds bars) and a coconut mousseline buttercream frosting.
First, for the cake layers, used the Double Chocolate Layer Cake recipe at Epicurious. This is our standard chocolate cake recipe, with only a few changes/substitutions for the Mounds Cake: 1.) I always use freshly-brewed espresso instead of coffee, and 2.) this time I used 3/4 c. coconut oil instead of vegetable oil and 3.) I didn't have buttermilk on hand, so I mixed together a bit of cream and a bit of skim milk, added some lemon juice and let it stand for about 5 minutes to thicken. I used this thick curdled cream in lieu of the buttermilk.
Ignore the recipe for the ganache that accompanies the cake! Bake the layers the day before serving, allow to cool, and then wrap tightly in plastic wrap. This seals in moisture so the cake is fudgy and tender.
For the filling: I used this recipe for homemade Mounds bars as my model. But instead of using canned sweetened condensed milk, I made my own and added coconut cream to imbue it with coconut flavor.
To begin, I used this recipe to make homemade confiture de lait (or sweetened condensed milk), but I halved the recipe and used cream instead of milk.
So: I combined 3/4 c. cream, 1/3 c. sugar, and 1/2 tsp vanilla extract in a heavy-bottomed saucepan. I heated it to a simmer over low heat (patience is key here, so it doesn't burn!) and cooked it, whisking often to prevent sticking/burning, for 2 or 2 1/2 hours. When the mixture had reduced by half, I stirred in1 1/2 Tbl. of the coconut cream paste (instead of butter). Then I put this in the fridge to chill. Put the rest of the coconut cream paste in a container and refrigerate it as well; you will pull it out again when you make the frosting.
Once the confiture had chilled, I spooned it into the bowl of my food processor. I added approximately an equal amount of unsweetened flaked dried coconut and pulsed them together to make a smooth paste. At first it was a little too sticky and thin, so I added more coconut (a little bit at a time) until I had made something which resembled the filling of a Mounds candy bar.
above: the confiture de lait, mixing up the filling.
I centered one of my cake layers on my cake stand, and began spreading the filling over the top of it. Or rather, I kind of patted it out. The filling was so thick that it was like a sticky dough, so I patted and pinched it between my fingers to make a disc, then laid that disc on top of the cake and gently poked and pushed at the edge to spread it around. Because the filling was very sticky, I had to be careful not to pull up bits of cake from the top of the cake layer as I worked it around. Once I had the filling pressed evenly around the layer, I placed my second cake layer on top of the filling. I pressed down gently, to kind of seal the two layers together with the filling. Then I put three wooden skewers into the cakes to hold it together for frosting, and put the cake into the fridge to chill it. Chilling the cake made the filling firm up and become very solid, which helped hold the whole thing together. Chilling a cake also helps prevent it from causing frosting to melt when you apply the first "crumb coat" to the cake.
above: the disc of coconut-cream filling has been patted and prodded out to the edges of the bottom layer; the top layer is pressed onto the filling, and skewers hold the cake layers in place.
MOUSSELINE BUTTERCREAM FROSTING:
While the cake chills, it's time to make mousseline (or Italian Buttercream) frosting. I used this recipe from The Kitchn as my model, with a few tweaks to give it a rich coconut flavor. Why bother making a fussy Italian buttercream, you might ask? Well, when making a birthday cake for the middle of summer, I find that a standard American buttercream has a tendency to melt and run. This cooked meringue base promised to hold together better in heat, and wouldn't be as sweet as a fondant.
To make my buttercream, I separated six eggs. I saved the yolks for another use and let the whites stand out at room temp for 30-60 minutes before beginning to make my frosting. At this point, I also got the remaining coconut cream out of the fridge, to start warming up and softening it.
Using a hand-held mixer, I beat the whites with a pinch of salt in the bowl of my stand mixer until soft peaks were formed. Then I transferred the bowl back to my stand mixer.
Meanwhile, on the stove, I had combined 1 cup of sugar and 1/2 cup of water with 1/4 tsp of Cream of Tartar in a saucepan. I heated this mixture over medium heat until it reached the soft ball stage (between 235 and 240 degrees Fahrenheit). I actually got out a glass of ice water to check the accuracy of my candy thermometer - and boy was I ever glad that I did. My thermometer was more than 10 degrees off!
Once the sugar syrup had actually reached soft ball stage, I took the sauce pan over to my stand mixer. I set the mixer running on high, and slowly poured in the syrup in a thin stream into the egg whites. After pouring in all of the syrup, I left the mixer running. According to The Kitchn's recipe, you must let the mixer keep mixing until the sides of the bowl feel "room temp" when you touch them (so it's a good idea to use a metal bowl for this, or something which conducts heat well).
While the mixer was beating the meringue up to fluffy whiteness, I prepped my 2 cups of butter. Only I wasn't going to use two cups of butter: can you guess my secret? Yes, the remaining coconut cream. Measure out the coconut cream (which will probably still be hard; that's okay). I think I had about a half cup left. Combine with enough butter to make two cups. Soften by microwaving for about 5 seconds at a time. As the butter softens, use a fork to mash it into the coconut cream and mix the two together.
Once the meringue has reached room temp, reduce the blender's speed to medium and start adding the butter/coconut cream, one tablespoon at a time. After all the butter/coconut is incorporated, add 1 Tbl vanilla extract (or you could use coconut here; I just didn't have any on hand). Chill the buttercream until firm before applying to cake.
above: pouring hot sugar syrup into egg whites; mashing coconut cream together with butter to make a smooth, creamy paste
Once the buttercream has chilled to a spreadable consistency, it's time to put your first coat of frosting on the cake. This is called a "crumb coat," and the benefit of a crumb coat is that it kind of seals the cake together, so that you won't drag crumbs or bits of cake through your frosting when you do the final coat. The crumb coat should be as thin as you can manage. After frosting the cake with the crumb coat, chill the cake again for an hour to make sure the frosting is firm.
above: chilled cake layers and frosting. Do you notice the granular quality of my frosting? It was a little too cold to use. I let it warm up a bit more and whipped it up with my handheld blender and it became smooth again. At right: the crumb coat is on! Leave the wooden skewers in the cake until you are applying the second/final coat of frosting (and even then, frost the sides of the cake first). They will help provide stability, so nothing slides or slips while you are trying to frost the cake.
After applying the final coat of frosting, chill the cake again until you are ready to serve. Then, enjoy!