Preheat oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
In a bowl, whisk together flour, baking powder, and salt.
In another bowl, beat eggs and granulated sugar until thick and pale. Add melted butter, milk, and vanilla extract. Mix well.
Gradually add dry ingredients to wet ingredients, mixing until just combined. Divide batter between prepared pans.
Bake for 20-25 minutes or until a toothpick inserted into the center comes out clean. Cool in pans for 10 minutes, then transfer to wire racks to cool completely.
Mix espresso and coffee liqueur in a small bowl. Set aside.
In a large bowl, beat mascarpone cheese, heavy cream, and powdered sugar until stiff peaks form.
Place one cake layer on a serving plate. Brush with half of the espresso mixture. Spread half of the mascarpone mixture over the cake.
Place the second cake layer on top. Brush with remaining espresso mixture. Spread remaining mascarpone mixture over the top and sides of the cake.
Dust with cocoa powder before serving. Chill for at least 2 hours before slicing.