Preheat your oven to 350°F (177°C). Grease and line two 6-inch round cake pans with parchment paper and set aside.

In a medium bowl, sift together cake flour, baking powder, and salt, then set aside.

Combine instant espresso powder with boiling water in a small cup, stirring until dissolved; allow to cool.

Cream softened unsalted butter in a large bowl until creamy. Gradually add sugar and mix until fluffy, about 3-4 minutes.

Add vanilla extract, then eggs one at a time, mixing well. Alternate adding dry ingredients and milk until just combined, then fold in cooled espresso.

Divide batter between prepared pans and bake for about 20 minutes, checking for doneness.

Allow the cakes to cool in pans for 5 minutes, then transfer to wire racks to cool completely.

Whip mascarpone cheese until creamy, gradually adding powdered sugar and coffee, then fold in heavy cream.

Slice each cooled cake horizontally to create layers, then soak layers with coffee or liquor and frost between layers and around the outside.

Dust top of cake with cocoa powder and serve.