1. Preheat oven to 350 degree F and line three 12-cup muffin pans with foil or paper muffin liners.
2. Whisk together water and cocoa powder in a medium bowl until combined well. Add chocolate and let stand for 5 minutes. Whisk mixture until chocolate is melted and smooth. Whisk in buttermilk and vanilla.
3. Whisk together flour, baking soda, and salt in another bowl.
4. Beat together butter and sugar in a large bowl with an electric mixer until light and fluffy. Add eggs 1 at a time, beating well after each addition, until combined well. On low speed, add flour mixture and coca mixture alternately in batches, beginning and ending with flour mixture, until just combined.
5. Pour batter evenly among cups until three-quarters full (about a heaping 1/4-cup measure) and bake in batches in upper and lower thirds of oven, switching position of pans halfway through, until a toothpick inserted in center comes out clean, 18 to 20 minutes. Let cupcakes cool slightly in pans on racks, then remove cupcakes from pans.Make Frosting::
6. Melt white chocolate with heavy cream in a heat-proof bowl set over a saucepan of simmering water, stirring until smooth, about 8 minutes. Transfer bowl to refrigerator and chill until thick but still pourable, about 1-1/2 hours. Beat in butter and food coloring with an electric mixer until soft and fluffy. Immediately pipe frosting onto cupcakes with a pastry bag fitted with a 1/2-inch-wide star tip. Makes 2-1/2 dozen.