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