For the Chocolate-Malt Buttercream:
To Make the Cake:
1. Whisk together the flour, cocoa, baking powder, baking soda and salt.
2. Working with a stand mixer, preferably fitted with a paddle attachment, or with a hand mixer in a large bowl, beat the butter on medium speed until soft and creamy. Add the sugar and beat for about 2 minutes, until it is thoroughly blended into the butter. Add the eggs one at a time, then the yolks one by one, beating for 1 minute after each addition and scraping down the sides and bottom of the bowl as needed. Beat in the vanilla. Reduce the mixer speed to low and add the dry ingredients alternately with the buttermilk; add the dry ingredients in 3 portions and the buttermilk in 2 (begin and end with the dry ingredients); mix only until each new batch is blended into the batter. Scrape down the bowl and, if you want, add the melted chocolate, folding it in with a rubber spatula. Divide the batter between the cake pans.
3. Bake for 26 to 30 minutes, or until the cakes feel springy to the touch and start to pull away from the sides of the pans. Transfer the cakes to racks and cool for about 5 minutes, then run a knife around the sides of the cakes, unmold them and peel off the paper liners. Invert and cool to room temperature right side up. (Once the layers are cooled, they can be wrapped airtight and left at room temperature overnight or kept frozen for up to 2 months.)
To Make the Buttercream:
1. Melt the chocolate with half the brown sugar in a heatproof bowl set over a saucepan of simmering water. Remove from the heat.
2. Whisk the malt powder and cocoa together in a small bowl, pour over 3 tablespoons of the boiling water and whisk until smooth. Whisking the melted chocolate gently, gradually pour in the hot malt-cocoa mixture and stir to blend—it should be dark, smooth and glossy; set aside.
3. Working with the stand mixer, preferably fitted with the paddle attachment, or with a hand mixer in a large bowl, beat the butter on medium speed until soft and fluffy, about 2 minutes. Add the remaining brown sugar and beat for 2 to 3 minutes more, until well blended. Beat in the salt and vanilla extract, then reduce the mixer speed to low. Scrape in the chocolate mixture and mix until smooth. Still working on low speed, gradually add the confectioners sugar. When all the sugar is in, increase the mixer speed to medium and beat for a couple of minutes. Lower the speed and add the remaining tablespoon of boiling water, then increase the speed and give the frosting another quick spin. It will be light and should be thick enough to use immediately. If it doesn't hold its shape, beat it just a bit more.
To Assemble the Cake: Place one layer top side up on a cardboard round or a cake plate protected by strips of wax or parchment paper. Frost the top of the layer, and cover with the second layer, top side down. Frost the sides and top of the cake, either smoothing the buttercream for a sleek look or using a spatula, knife or spoon to swirl it for a more exuberant look.
Refrigerate the cake for at least 1 hour (or for up to 1 day, if that's more convenient) to set the frosting, then bring it to room temperature before serving.
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