Preheat the oven to 325 degrees. Coat two 9-inch cake pans with cooking spray and line the bottoms of each with parchment paper. Spray parchment paper with cooking spray as well. Dust both pans with flour, tapping out the excess.
In a medium saucepan, melt butter. Cook over low heat, stirring occasionally, until the milk solids turn brown and the butter smells nutty. Because it is a low heat, the process will take about 20 minutes. Scrape the melted butter and browned bits into a large heatproof bowl. Set the bowl in an ice water bath until the butter begins to solidify around the edge, approximately 8 minutes.
In a medium mixing bowl whisk 2 1/4 cups of all-purpose flour with the baking powder and salt.
Remove the bowl from the ice water and scrape up the hardened butter. Transfer the butter to a room temperature bowl and beat until creamy, 3-4 minutes, the color of the butter will lighten as you beat. Add the sugar and vanilla pods (or extract) and beat at medium-high speed until fluffy, about 3 minutes. Beat in egg yolks followed by the whole eggs. Beat in dry ingredients and milk in 3 alternating additions.
Pour the cake batter into prepared pans and bake in the center of the oven for about 40 minutes, rotating the pans halfway through baking, until the cakes are golden and they pass the toothpick test. Cool the cakes in the pans for 20 minutes. Invert them onto a rack to let them cool completely. Peel off the parchment paper.
Assemble Dark Chocolate Mousse filling and Chocolate Buttercream Frosting, see instructions below.
Set one cake layer on a platter. Spread the mousse filling on top, sprinkle with powdered sugar, and cover with the second cake layer. Frost the outside of the cake with buttercream, sprinkle powdered sugar.
Dark Chocolate Mousse Filling
In a small bowl, sprinkle gelatin over the water and let stand until softened.
Meanwhile, in a medium bowl beat cream until softly whipped. Refrigerate until chilled, about 10 minutes.
In a small, microwave-safe bowl, heat coffee liqueur at high power until hot, about 45 seconds. Stir in softened gelatin until dissolved. You might need to use a spatula to scrap up gelatin as it tends to stick to the bowl and peel off.
In another medium bowl, beat the egg yolks with sugar and salt at high speed until pale and thickened, 5 minutes. While beating yolks, beat in coffee-gelatin liquid. Beat in melted chocolate. Using a rubber spatula, fold in whipped cream in 2 additions. Scrape mousse into a bowl, cover with plastic wrap and refrigerate or freeze until firm enough to spread easily. If you refrigerate too long or make this portion ahead of time, let stand at room temperature to soften. Do NOT microwave.