Preheat oven to 350°F. Prepare a 9-inch round cake pan with parchment paper and grease the sides.

In a bowl, whisk together flour, baking powder, and salt. Set aside.

Cream the butter and sugar until light and fluffy, then add the egg, vanilla, and milk. Mix well.

Dissolve espresso powder in hot water, then add Kahlua and set aside.

Mix mascarpone, cream cheese, and sugar until smooth. Add sour cream, vanilla, eggs, and yolks, and beat until combined.

Layer ladyfingers soaked in espresso mixture in the cake pan, then spread with cheese mixture, repeating layers.

Bake at 300°F for 1 hour 15 minutes, then turn off oven and leave door closed for 30 minutes.

Refrigerate until thoroughly chilled, then top with cocoa powder before serving.