Make the cupcakes: Preheat the oven to 375 degrees F and coat a 12-cup mini-muffin pan with cooking spray. Sift the flour, cocoa powder and salt into a small bowl.
Beat the egg yolk and 2 tablespoons granulated sugar in a medium bowl with a mixer on medium-high speed until light and thick, about 2 minutes. Beat in 2 teaspoons coffee and the vanilla. Fold in the flour mixture with a rubber spatula until just incorporated.
Beat the egg white in a separate bowl with clean beaters on medium speed until foamy. Increase the speed to medium high and gradually add the remaining 1 tablespoon granulated sugar; beat until stiff shiny peaks form, about 3 minutes.
Stir about one-third of the beaten egg white into the batter with a rubber spatula, then gently fold in the rest. Divide the batter evenly among the mini-muffin cups, filling them halfway. Bake until the cupcakes are golden brown and spring back when pressed, about 10 minutes. Poke a few holes into each with a toothpick, then brush with the remaining 1 tablespoon coffee. Remove to a rack and let cool completely.
Make the frosting: Beat the butter and confectioners' sugar in a large bowl with a mixer on low speed until combined, then increase the speed to medium high and beat until light and fluffy, about 2 minutes; add the mascarpone and beat until just incorporated. Beat in the vanilla.
Spread the coconut on a plate. Cover each cupcake completely with a thick layer of frosting, then roll in the coconut. Refrigerate at least 30 minutes and up to 1 day.