01 - Preheat oven to 350°F. Grease and line two 8-inch round cake pans with parchment paper.
02 - Beat eggs and sugar in a large bowl until light and fluffy, about 3-4 minutes. Add butter and vanilla extract; mix well.
03 - Whisk together flour, baking powder, and salt in a separate bowl.
04 - Gradually add dry ingredients to the wet mixture, alternating with milk. Mix until just combined.
05 - Divide batter evenly between prepared pans. Bake for 22-25 minutes or until a toothpick inserted into center comes out clean.
06 - Let cakes cool in pans for 10 minutes, then turn out onto a wire rack to cool completely.
07 - Stir together espresso, coffee liqueur if using, and sugar until the sugar dissolves.
08 - Whip heavy cream to stiff peaks in a chilled bowl. In another bowl, beat mascarpone, powdered sugar, and vanilla until smooth. Gently fold whipped cream into mascarpone mixture until fully combined.
09 - Slice each cake layer horizontally for a total of four layers. Place one layer on a serving plate and brush generously with coffee syrup.
10 - Spread a quarter of the mascarpone filling over the cake. Repeat layering with remaining cake, syrup, and filling, ending with a mascarpone layer on top.
11 - Cover and chill the assembled cake at least 4 hours or overnight for best flavor.
12 - Dust the top with cocoa powder and optionally garnish with chocolate shavings before serving.