01 - Preheat oven to 350°F. Grease two 8-inch round cake pans and line with parchment paper.
02 - Beat eggs and sugar in a large bowl until light and fluffy, about 3 to 4 minutes. Add softened butter and vanilla extract; mix well.
03 - Whisk together flour, baking powder, and salt in a 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 for 22 to 25 minutes until toothpick inserted in center comes out clean.
06 - Let cakes cool in pans for 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. Beat mascarpone, powdered sugar, and vanilla until smooth. Fold whipped cream into mascarpone mixture until combined.
09 - Slice each cake layer horizontally for total of four layers.
10 - Place one layer on serving plate. Brush generously with coffee syrup. Spread quarter of mascarpone filling over cake.
11 - Repeat layering with remaining cake, syrup, and filling, ending with mascarpone layer on top.
12 - Cover and refrigerate at least 4 hours or overnight. Dust top with cocoa powder and garnish with chocolate shavings before serving.