Cream together softened butter and sugar until fluffy, about 2–3 minutes. Beat in eggs one at a time. In another bowl, whisk together flour, baking powder, and salt. Mix together milk and cooled espresso. Alternate adding dry ingredients and liquid into the butter mixture until just combined.
Line a muffin tin with liners and fill them about two-thirds full with batter. Bake at 350°F (175°C) for approximately 15–20 minutes or until a toothpick inserted comes out clean. Allow to cool in the pan for about five minutes before transferring to a wire rack.
Heat espresso with sugar until dissolved. Lightly brush this mixture over warm cupcake tops.
Beat mascarpone cheese until smooth. Gradually drizzle in cold heavy cream while whipping until soft peaks form. Add powdered sugar and whip until stable soft peaks are achieved.
Once cupcakes have cooled, pipe or spread frosting on top. Dust with cocoa powder and garnish with chocolate curls or coffee beans.