Preheat oven to 350°F Line 2 muffin pans with muffin cups or coat with nonstick spray.
In a medium, heatproof bowl, combine cocoa powder with chocolate pieces. Whisk in the boiling water until chocolate is melted and mixture is combined.
In a large bowl, cream butter and granulated sugar together. Beat in the eggs one at a time, until fully blended. Sift in flour, baking powder, and salt, and mix until just combined.
Alternating, add the chocolate mixture and wine in batches, mixing well between each addition. Repeat this until batter is smooth. Divide mixture among muffin cups, filling each about 3/4 of the way full.
Bake until a toothpick inserted in the center comes out clean, about 20 minutes. Let cool in the pan.
Lightly beat the mascarpone, then add the confectioners' sugar and beat until creamy. Remove the cooled cupcakes from the pan, spread each with icing, and top with a grape half, if desired.