Begin by creaming the butter and sugar in a large mixing bowl until light and fluffy.

Add eggs one at a time, incorporating well.

In a separate bowl, whisk together flour, cornstarch, baking powder, baking soda, and salt.

Gradually alternate adding the dry mixture with sour cream, vegetable oil, and vanilla extract until just combined.

Preheat the oven to 350°F (175°C) and prepare cake pans.

Divide batter evenly between the pans and bake for 25-30 minutes until a toothpick comes out clean.

Cool the layers in the pans for 10 minutes, then invert them to cool completely on a wire rack.

Make the frosting by beating mascarpone and powdered sugar until smooth; add vanilla bean paste.

Whip heavy cream until stiff peaks form, then gently fold into the mascarpone mixture.

Assemble the cake by soaking layers with espresso mixture and spreading frosting in between.

Chill the assembled cake for at least 4 hours or overnight. Dust with cocoa powder before serving.