Preheat your oven to 350°F (177°C) and prepare two 6-inch round cake pans by greasing and flouring them.

Sift together the cake flour, baking powder, and salt in a medium bowl. Set aside.

Dissolve the instant espresso powder in boiling water and allow it to cool slightly.

Cream softened unsalted butter in a stand mixer until fluffy. Gradually add sugar until pale and airy.

Add vanilla extract and eggs one at a time, blending each until well combined.

Alternate adding the dry ingredient mixture and whole milk to the butter mixture until just combined.

Gently fold in the cooled espresso mixture into the cake batter.

Divide the batter evenly between the prepared cake pans and bake for about 20 minutes.

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

Whip mascarpone cheese until creamy, then blend in powdered sugar and coffee. Add heavy whipping cream and beat until smooth.

Level the cooled cake layers if necessary, moisten with liquor or coffee, spread frosting, and assemble.

Dust the top with cocoa powder and sprinkle with chocolate shavings, then let rest before serving.