Prep the cake materials: Center a rack in the oven and preheat the oven to 350 F. Butter two 9 x 2-inch round cake pans with a pastry brush, dust the interiors with flour, tap out the excess and line the bottoms with rounds of parchment or wax paper. Put the pans on a baking sheet.
For the cake: Whisk together the flour, cocoa, baking soda, baking powder, and salt. Set aside. Beat the butter until soft and creamy in a stand mixer, preferably fitted with a paddle attachment, or with a hand mixer in a large bowl. Add the sugar and beat until it is thoroughly blended with the butter, about 2 minutes, scraping down the sides of the bowl halfway through for even mixing. Add the eggs yolks one at a time, then the eggs one at a time, 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. Beginning with the dry ingredients, alternate adding the dry ingredients and the buttermilk; add the dry ingredients in 3 portions and the buttermilk in 2. Mix only until each new batch is blended into the batter (i.e., do not overmix). 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. Bake for 26–30 minutes, or until the cakes feel springy to the touch and start to pull away from the sides of the pans.
For the chocolate malt buttercream: Add the chocolate and half the brown sugar to a heatproof bowl; set the bowl over a saucepan of simmering water to temper (melt) the chocolate and sugar, making sure the bowl doesn't touch the water. Meanwhile, whisk the malt powder and cocoa together in a small bowl; add 3 tablespoons of boiling water and whisk until smooth. Set aside.
In 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 until well blended, about 2–3 minutes. Beat in the salt and vanilla extract, then reduce the mixer speed to low.
When the chocolate and sugar are melted, remove heatproof bowl from the saucepan. Whisking the melted chocolate gently, gradually pour in the malt-cocoa mixture and stir to thoroughly incorporate: it should be dark, smooth, and glossy. Add this chocolate mixture to the stand mixer and mix until smooth, pausing to scrape down the bowl. Still working on low speed, gradually add the confectioners' sugar. When all the sugar is in, raise the mixer speed to medium and beat for a couple of minutes. If the frosting is too thin, pop it in the fridge for a few minutes.
Remove the cakes from the oven; transfer to racks and cool for about 5 minutes. 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. (Note: Once the layers are cooled, they can be wrapped airtight and left at room temperature overnight or kept frozen for up to 2 months.)
Assemble the cake: Place one layer on a cardboard round or a cake plate protected by strips of wax or parchment paper. Frost the top of the layer, then cover with the second layer of cake. Frost the top and sides (optional) of the cake, smoothing the buttercream for a sleek look or using a spatula, knife, or spoon to swirl it for a more exuberant look. Remove the paper strips after the cake is frosted. 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.