Preheat the oven to 350ºF. Lightly grease two 8-inch round cake pans with baking spray and line the bottoms with parchment paper. I suggest wrapping the pans in soaked fabric baking strips for even, flat layers.

In a large liquid measuring cup or medium bowl, whisk together the milk, sour cream, egg whites, and vanilla extract. Set aside.

In the bowl of a stand mixer fitted with the paddle attachment, mix together the flour, sugar, baking powder, and salt. While mixing on low speed, add the butter 1 tablespoon at a time until the mixture looks like coarse crumbs, 30 seconds to 1 minute.

With the mixer running on low, drizzle half of the milk mixture into the mixer, and mix just until it is fully combined. Stop and scrape the bowl down, then turn the mixer back on low and mix in the rest of the milk mixture. Beat on medium speed until the mixture looks fluffy, about 30 seconds. Scrape down the bowl and mix once more if needed.

Divide the batter evenly between the two pans. Bake for 35 minutes or until a toothpick inserted comes out clean. Let the cakes cool in the pans for 15 minutes, then invert them onto a wire rack, remove the parchment paper on the bottom, and cool completely.

Place the milk in a small pot over medium heat and warm until the milk starts to steam and bubble slightly around the edges, about 2-3 minutes.

Meanwhile, in a medium bowl, whisk together the egg yolks, sugar, and cornstarch until smooth. Set aside.

Once the milk is hot, slowly whisk it into the egg yolk mixture until fully combined. Pour the mixture back into the saucepan and cook over medium-low heat, gently whisking constantly, until thickened, about 2 minutes. (The custard should hold the trace of the whisk when pulled through it and temp around 200°F.)

Remove from heat and stir in the coffee liquor. Transfer the custard to a clean bowl, and press plastic wrap on top. Chill for 30 minutes to 1 hour, just until it is roughly room temperature (70-80F). To quickly cool it, you can put the bowl in an ice bath and stir the custard with a spatula until cooled, about 5 minutes.

Once cooled, gently whisk the mascarpone in a medium bowl until smooth and fold half of the custard into it. Add the remaining half and and fold just until no streaks remain, cover and chill until ready to use.

Add the egg whites, sugar, and salt to a bowl that is large enough to sit on the pot of simmering water without touching the water. Give the mixture a brief whisk. Place the bowl on top of the pot. Gently stir the egg whites constantly, until they have reached 160F, about 8-10 minutes.

Transfer the egg white mixture to the bowl to a stand mixer fitted with a whisk attachment. Beat on medium speed until the mixture becomes very fluffy, glossy, and has soft peaks, and the outside of the bowl feels barely warm to the touch, about 10-12 minutes.

Reduce the mixer speed to medium-low, and add the softened butter 1 tablespoon at a time, making sure to let each piece of butter incorporate before you add the next one.

Once the butter is incorporated, add the coffee liqueur and vanilla into the buttercream. Increase the mixer speed to medium-high and beat until the frosting is fluffy and stiff enough to spread, about 2-3 minutes. Stop and scrape the bowl down occasionally, as well. (If your frosting is runny at this stage, see the Notes for how to troubleshoot it by dropping the temperature.)

Place ¾ cup (128g) of buttercream in a piping bag fitted with a large closed star tip (I used Ateco 846 tip). Place another 1 cup of buttercream in a piping bag fitted with a round tip (1A). Cover the rest of the frosting, and set aside.

In a small bowl, stir together the water, espresso powder and coffee liqueur. Set the syrup aside to cool slightly.

Cut the cooled cake layers in half horizontally, so you have four thin layers. Transfer the cooled, mascarpone custard to a piping bag.

Place the first layer, cut side up, on a cake stand. Drizzle or brush a third of the coffee syrup (about 5 tablespoons) on 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. Dollop a third of the mascarpone filling (1 cup) on top and spread it into a smooth layer out to the edge of the cake. Dust the top lightly with cocoa powder.

Repeat again twice by starting with the cake layer cut side up, then adding the coffee syrup, frosting, mascarpone filling, and cocoa powder. For the final top cake layer, place it bottom side up for a smooth top.

Cover the top and sides with a smooth layer of the buttercream. (You can scrape the side for a “naked” look, but if you love buttercream leave it as a nice thick layer!)

Dust top with cocoa powder. Use the end of a spoon to remove a ring of cocoa powder from the edge of the cake’s top (this will allow your dollops to adhere). Pipe dollops of buttercream in the clean ring around the top, and garnish with chocolate covered coffee beans. Let the cake chill in the refrigerator for at least 1 hour before serving.