Preheat the oven to 175°C. Grease three 20 cm round cake pans, line the bottoms with parchment paper, and lightly flour the sides.
In a bowl, whisk together the flour, baking powder, and salt. Set aside.
In a large mixing bowl, beat the butter and granulated sugar until pale and fluffy, about 4–5 minutes.
Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
Add the dry ingredients in three additions, alternating with the milk, beginning and ending with the dry mixture. Mix just until smooth.
Divide the batter evenly between the prepared pans and level the tops.
Bake for 25–30 minutes, or until a toothpick inserted in the center comes out clean.
Cool the cakes in the pans for 10 minutes, then turn out onto a wire rack and cool completely.
To make the coffee soak, stir the sugar into the cooled espresso until fully dissolved. Set aside.
In a large chilled bowl, whip the heavy cream with the powdered sugar and vanilla until soft, stable peaks form.
In a separate bowl, gently loosen the mascarpone with a spatula.
Fold the whipped cream into the mascarpone in two additions, mixing slowly to keep the cream light and smooth.
Level the tops of the cooled cake layers if needed.
Place the first layer on a serving plate or cake stand and brush generously with the coffee soak.
Spread a thick, even layer of mascarpone cream over the top.
Add the second cake layer, soak it with coffee, and repeat with another layer of cream.
Place the final cake layer on top and brush with the remaining coffee soak.
Apply a thin crumb coat of mascarpone cream around the cake, then add a final smooth layer, leaving the sides slightly exposed for a refined, semi-naked finish.
Transfer the remaining cream to a piping bag fitted with a round tip and pipe small dollops across the top of the cake.
Just before serving, dust the top generously with sifted cocoa powder.
Chill the cake for at least 4 hours, or overnight, to allow the flavors to meld and the structure to set.
