Chocolate Cake Part 2: The Buttercream

We interrupt today's normal "$10 Mondays" posts for something much more important: buttercream. I first attempted buttercream a couple of years ago, and it has since become my favorite frosting, although it is certainly more work than the usual method of creaming butter and sugar. However, I think it is worth the effort and in this case, the lightness of the buttercream offsets the dense and rich cake very nicely. I do, however, have a confession to make: I was so distracted on the day of Ari's party that I didn't even realize I had taken out the 1/3 measuring cup instead of the 1/2 -- thus, my total amount of sugar was 1 1/4 cup, rather than 1 1/2 cups, as specified in the recipe. I actually found that it was plenty sweet and didn't realize my mistake until later on, so I will include both measurements below. The recipe comes from Ad Hoc by Thomas Keller, probably my all-time favorite cookbook and a master of technique and instruction. I also think that next time I would make all Nutella buttercream, rather than dividing it into two batches, as the flavor wasn't discernibly different.

Chocolate and Nutella Buttercream (adapted from Ad Hoc)

1 3/4 cup + 2 tablespoons sugar (I used 1 1/4 cup + 2 tablespoons)

6 egg whites

3 1/2 sticks (14 oz) unsalted butter, chilled and cut into 1-inch pieces

5 oz milk chocolate, melted

3 tablespoons Nutella, softened


Fill a medium saucepan halfway with water and bring to a simmer over medium heat.  Combine the sugar and egg whites in a large metal bowl, set over the simmering water, and whisk constantly until the sugar has dissolved and the mixture is hot to the touch (150-160 degrees). This will take about 10 minutes.
Remove the bowl from the heat and using an electric mixer, whip on high speed until the whites are thick and hold stiff peaks and the whites and the bowl are cool to the touch, 10 to 12 minutes.
Lower the speed to medium, and add the butter 2 to 3 pieces at a time, being sure each batch is incorporated before adding more.  If at any point the mixture looks broken or curdled, increase the speed to medium-high and beat to bring it back together, then reduce the speed and continue adding the butter. Once all of the butter has been added, divide the buttercream into two batches (about 3 cups and 2 cups). Mix the smaller batch with Nutella, and the larger batch with the melted chocolate.
To frost the cake: place the first layer, top-side down, on a large plate. Using an offset icing spatula, spread 1/2 of Nutella buttercream on top, so that it is about 1/2 inch thick. Place next layer, flat side down, on top of frosting. Spread rest of Nutella buttercream on top, and place last layer on top of that. Begin spreading chocolate buttercream around the edges of the cake, filling in gaps and smoothing as you go along.
Repeat with the top of the cake and make as smooth as possible. Decorate as desired. (I piped melted chocolate for the lettering). Refrigerate until ready to serve.