Adjust the oven rack to the middle position and preheat oven to 350°F. Line a standard cupcake pan with cupcake liners. In a small bowl, whisk together the chocolate and coffee until the chocolate is melted. Set aside.
2. Sift together the flour, cocoa, baking soda, baking powder, and salt into a medium bowl. In a large bowl, whisk together the sugar, egg, vegetable oil, buttermilk, and vanilla until combined. Whisk in the
chocolate-coffee mixture until combined. Whisk in the dry mixture until combined.
3. Divide batter equally among the cupcake cups and bake until just set and a toothpick inserted in the center comes out clean,
17 to 20 minutes. Let cool in pan for 5 minutes, then transfer to a wire rack to cool completely.
4. Using an offset spatula, cover each cupcake with a generous layer of frosting, using swirling motions to apply it. Sprinkle with the sanding sugar. Arrange on a tray lined with the rock candy crystals and set on
the table.