Preheat oven to 350°F and line a 12-cup muffin pan with cupcake liners.In a medium bowl, whisk together flour, baking powder, and salt. Set aside.In a large bowl, beat butter and sugar on low speed until blended. Increase to high and whip for 3 minutes until light and fluffy. Scrape the sides of the bowl.Add the egg and beat until combined. Add egg yolks and vanilla extract, mixing well. Scrape the bowl again.Alternate adding the dry ingredients with the milk and buttermilk in three batches, starting and ending with the flour. Mix just until combined after each addition.Divide half the batter into a separate bowl and fold in the espresso powder. It may not fully dissolve—this is okay.Scoop about 2 tablespoons of vanilla batter into each cupcake liner, followed by 2 tablespoons of espresso batter.Bake for 18 to 22 minutes or until a toothpick inserted in the center comes out clean. Let cool in the pan briefly, then transfer to a wire rack to cool completely.To make the frosting, beat mascarpone and powdered sugar until smooth. Add cream and whip to stiff peaks. Mix in vanilla extract.Frost the cooled cupcakes with a piping bag or spatula. Dust with cocoa powder before serving.Store cupcakes in the refrigerator and enjoy within 2 days for the best texture and flavor.