Make the cake:
Preheat the oven to 350°F (175°C). Grease and flour two 8-inch round cake pans. In a medium bowl, whisk together flour, baking powder, and salt. In a large bowl, cream together the butter and sugar until light and fluffy. Add eggs one at a time, mixing well after each addition. Stir in vanilla extract. Gradually add the dry ingredients, alternating with coffee (and coffee liqueur, if using), beginning and ending with the dry ingredients. Mix until just combined.

Bake the cake:
Divide the batter evenly between the two cake pans and smooth the tops. Bake for 25–30 minutes, or until a toothpick inserted into the center comes out clean. Let the cakes cool in the pans for 10 minutes, then turn them out onto a wire rack to cool completely.

Make the mascarpone filling:
In a medium bowl, beat the mascarpone cheese, heavy cream, powdered sugar, and vanilla extract until smooth and fluffy, about 2-3 minutes.

Prepare the soaking syrup:
In a small bowl, combine brewed coffee, coffee liqueur (if using), and sugar. Stir until the sugar is dissolved.

Assemble the cake:
Once the cakes are completely cooled, slice each cake in half horizontally to create four layers. Place one layer of cake on a serving plate or cake stand. Brush generously with the coffee soaking syrup. Spread a layer of mascarpone filling on top. Repeat with the remaining layers of cake, syrup, and mascarpone filling, finishing with a final layer of filling on top.

Chill the cake:
Refrigerate the cake for at least 2–3 hours (or overnight) to allow the flavors to meld and the cake to firm up.

Finish and serve:
Before serving, dust the top of the cake with unsweetened cocoa powder and garnish with chocolate shavings, if desired.