Preheat the oven to 350 degrees . Line 2 muffin pans with baking liners.
In a medium, heatproof bowl, combine the cocoa powder and chocolate pieces. Whisk in the boiling water until the chocolate is melted and the mixture is combined.
In a large bowl, cream the butter and granulated sugar. Beat in the eggs, one at a time, until blended. Sift in the flour, baking powder and salt and mix until just combined. Alternating, add the chocolate mixture and wine in batches, mixing well between each addition, until the batter is smooth. Divide among the muffin cups, filling each about three-quarters 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.