Preheat the oven to 325 degrees F and place 6 lightly buttered oven-safe 4-ounce coffee cups on a rimmed baking sheet.
Sift the flour, baking powder and salt into a medium bowl and set aside. In the bowl of a stand mixer on medium-high speed, beat the eggs for 4 minutes. They will begin to lighten in color and become frothy. Add the granulated sugar and continue to beat for an additional 4 minutes. Reduce the mixer speed to low and stir in the flour mixture until just combined. Then mix in the melted butter, milk and 1/2 teaspoon of the vanilla. Divide the batter among the cups and bake until the tops are golden brown, lightly spring back to the touch and a toothpick inserted into the center comes out clean, 25 to 30 minutes.
While the cupcakes bake, mix together 1/4 cup of the confectioners' sugar, the liqueur, espresso powder, cocoa powder and hot water, and stir until dissolved.
When the cupcakes come out of the oven, poke 8 to 10 holes in each using a wooden skewer. Evenly but slowly spoon the coffee syrup over each cupcake. Let cool completely.
When the cupcakes are cool, beat the mascarpone with the heavy cream and the remaining 1/4 cup confectioners' sugar and remaining 1/4 teaspoon vanilla until soft peaks form. Divide among the cupcakes. Garnish each cupcake with a dusting of cocoa powder and/or chocolate shavings if using.