For the Cake:

1. Set the oven temperature to 350ºF. Prepare two 8-inch round cake pans by lightly greasing them with baking spray and lining the bottoms with parchment paper. For even layers, consider wrapping the pans with soaked fabric baking strips.
2. In a large liquid measuring cup or medium bowl, combine the milk, sour cream, egg whites, and vanilla extract by whisking them together. Set this mixture aside.
3. In the bowl of a stand mixer with a paddle attachment, blend the flour, sugar, baking powder, and salt. Gradually incorporate the butter, one tablespoon at a time, while mixing on low speed until the mixture resembles coarse crumbs, which should take about 30 seconds to 1 minute.
4. With the mixer on low, pour in half of the milk mixture and mix until just combined. Stop to scrape down the sides of the bowl, then add the remaining milk mixture. Increase the speed to medium and beat until the mixture appears fluffy, which should take about 30 seconds. Scrape down the bowl and mix again if necessary.
5. Evenly distribute the batter between the two prepared pans. Bake for 35 minutes or until a toothpick inserted in the center comes out clean. Allow the cakes to cool in the pans for 15 minutes, then invert them onto a wire rack, remove the parchment paper, and let them cool completely.

For The Filling:

1. In a small saucepan over medium heat, warm the milk until it begins to steam and bubble slightly around the edges, which should take about 2-3 minutes.
2. While the milk is heating, whisk together the egg yolks, sugar, and cornstarch in a medium bowl until smooth. Set this mixture aside.
3. Once the milk is hot, gradually whisk it into the egg yolk mixture until fully combined. Return the mixture to the saucepan and cook over medium-low heat, whisking gently and constantly until it thickens, which should take about 2 minutes. The custard should hold the trace of the whisk when pulled through it and reach a temperature of around 200°F.
4. Remove the saucepan from the heat and stir in the coffee liqueur. Transfer the custard to a clean bowl and cover it with plastic wrap, pressing it directly onto the surface. Chill for 30 minutes to 1 hour until it reaches a temperature close to room temperature (70-80°F). To cool it quickly, you can place the bowl in an ice bath and stir with a spatula until cooled, which should take about 5 minutes.
5. Once cooled, whisk the mascarpone in a medium bowl until smooth, then fold in half of the custard. Add the remaining custard and fold until no streaks remain. Cover and refrigerate until ready to use.

For the Buttercream:

1. In a bowl large enough to sit over a pot of simmering water without touching it, combine the egg whites, sugar, and salt. Briefly whisk the mixture, then place the bowl over the pot. Stir the egg whites gently and continuously until they reach 160°F, which should take about 8-10 minutes.
2. Transfer the egg white mixture to a stand mixer fitted with a whisk attachment. Beat on medium speed until the mixture becomes very fluffy, glossy, and forms soft peaks, and the bowl feels barely warm to the touch, which should take about 10-12 minutes.
3. Lower the mixer speed to medium-low and incorporate the softened butter one tablespoon at a time, ensuring each piece is fully mixed in before adding the next.
4. After all the butter is added, mix in the coffee liqueur and vanilla. Increase the speed to medium-high and beat until the frosting is fluffy and stiff enough to spread, which should take about 2-3 minutes. Stop occasionally to scrape down the bowl. (If the frosting is runny at this stage, refer to the Notes for troubleshooting tips.)
5. Place ¾ cup (128g) of buttercream into a piping bag fitted with a large closed star tip (I used Ateco 846 tip). Place another cup of buttercream in a piping bag with a round tip (1A). Cover the remaining frosting and set it aside.

For the Assembly:

1. In a small bowl, combine the water, espresso powder, and coffee liqueur. Allow the syrup to cool slightly.
2. Slice the cooled cake layers in half horizontally to create four thin layers. Transfer the cooled mascarpone custard into a piping bag.
3. Position the first layer, cut side up, on a cake stand. Drizzle or brush a third of the coffee syrup (approximately 5 tablespoons) over the top. Spread a thin layer of buttercream on top, then pipe a thin border around the edge of the cake using the reserved frosting with the round tip, creating a dam for the filling. Place a third of the mascarpone filling (1 cup) on top and smooth it out to the edges of the cake. Lightly dust the top with cocoa powder.
4. Repeat this process twice more, starting with the cake layer cut side up and adding the coffee syrup, frosting, mascarpone filling, and cocoa powder. For the final top layer, position it bottom side up for a smooth finish.
5. Apply a smooth layer of buttercream over the top and sides of the cake. (You can scrape the sides for a “naked” appearance, but if you prefer more buttercream, leave it thick.)
6. Dust the top with cocoa powder. Use the end of a spoon to create a ring of cocoa powder around the edge of the cake’s top to allow your dollops to stick. Pipe dollops of buttercream in the clean ring around the top and decorate with chocolate-covered coffee beans. Refrigerate the cake for at least 1 hour before serving.