Preheat the oven to 350°F (177°C) and prepare two 6-inch round cake pans by greasing and lining with parchment paper.

Sift together cake flour, baking powder, and a pinch of salt in a medium bowl.

Mix instant espresso powder with boiling water until fully dissolved and allow to cool.

In a large mixing bowl, beat softened unsalted butter with sugar until light and fluffy.

Add eggs, one at a time, mixing well, then blend in vanilla extract.

Gradually add sifted dry ingredients to the creamed mixture, alternating with whole milk until just combined.

Gently fold in cooled espresso mixture until evenly incorporated.

Divide batter between the pans and bake for approximately 20 minutes.

Cool cake layers in pans for 5 minutes, then transfer to wire racks to cool completely.

Whip chilled mascarpone cheese until smooth, and gradually mix in powdered sugar and heavy cream until thick.

Once cooled, slice each cake layer in half horizontally, soak if desired, and spread frosting between layers.

Dust the top with cocoa powder and chill in the refrigerator for at least 30 minutes before serving.

Let sit at room temperature for 20 minutes before slicing and serving.