How to Make It
Combine both chocolates in a large bowl. In a medium saucepan, combine the sugar and water and cook over moderate heat until dissolved. Boil without stirring, brushing down the sides of the pan with a wet pastry brush occasionally, until a deep-amber caramel forms, 8 to 10 minutes. Reduce the heat to low and carefully add the cream; it will bubble vigorously. Whisk until smooth, then pour the caramel over the chocolate. Let stand for 1 minute, then whisk until the frosting is smooth. Refrigerate until completely cool but spreadable, about 2 hours.
Preheat the oven to 350°. Line 18 muffin cups with paper liners. In a medium bowl, whisk the flour, cocoa powder, baking powder, baking soda and salt. In a measuring cup, combine the buttermilk and water. In a large bowl, beat the butter with both sugars until light and fluffy, about 3 minutes. Beat in the egg and vanilla. With the mixer on low speed, beat in the flour mixture in 3 additions, alternating with the buttermilk.
Spoon the batter into the cups, filling them three-quarters full. Bake for about 20 minutes, until a tester inserted in the center of a cupcake comes out clean. Cool in the pan for 10 minutes, then transfer the cupcakes to a rack to cool completely.
Meanwhile, spread the coconut on a rimmed baking sheet and toast for 2 to 3 minutes, until lightly golden. Let cool.
Frost the cupcakes, top with the toasted coconut and serve.