Preheat the oven to 350°F (175°C) and line a muffin tin with cupcake liners.

In a large mixing bowl, whisk together the all-purpose flour, cocoa powder, baking powder, and granulated sugar.

In another bowl, mix the eggs, milk, and vegetable oil until smooth and well blended.

Gently fold the wet ingredients into the dry mixture until just combined.

Spoon the batter into the prepared cupcake liners, filling each about two-thirds full. Bake for 18-20 minutes.

Remove from the oven and cool in the tin for about 5 minutes, then transfer to a wire rack to cool completely.

Whip the mascarpone cheese and espresso together until fluffy and combined.

Cut off the tops of the cooled cupcakes, scoop out some of the center, and fill with the mascarpone mixture. Replace the tops.

Dust the tops lightly with cocoa powder.