Preheat oven to 325 degrees. Line 14 cups of 2 standard (12-cup) muffin tins with foil liners. Melt chocolate in a heatproof bowl set over a pan of simmering water, stirring. Let cool completely. Sift flour, baking powder, and salt into a bowl; set aside.
Put butter and sugar into the bowl of an electric mixer fitted with the paddle attachment. Mix on medium-high speed until pale and fluffy, 3 to 5 minutes. Reduce speed to low. Mix in buttermilk and vanilla. Add flour mixture in 3 batches, alternating with the eggs and yolk. Add melted chocolate; mix until smooth.
Spoon batter into liners, filling about one-third full. Bake until a cake tester comes out clean, about 12 minutes. Let cool in tins on wire racks.
Assemble the cupcakes: Spread a thin layer of frosting on each cupcake. Using a 2 1/2-inch ice cream scoop dipped in hot water, place a smooth mound of frosting on top of each cupcake. Sprinkle with sanding sugar. Let stand until frosting is set, about 3 hours. Cupcakes can be stored in single layers in airtight containers at room temperature up to 1 day.