Preheat oven to 325 degrees. Line a 12-cup standard muffin tin with baking cups. Whisk together flour, baking powder, salt, and baking soda. In a heatproof bowl, whisk together sugar, cocoa, and chocolate. Whisk in cherry juice.

Melt butter in a small saucepan over medium heat, then continue to simmer until bubbles recede and solids on bottom of pan begin to turn golden brown, about 6 minutes. Pour into sugar mixture; whisk until well combined and chocolate has melted. Whisk in eggs, then dry ingredients.

Divide batter evenly among cups. Bake until tops spring back when lightly touched, 22 to 24 minutes. Let cool in tin 5 minutes; transfer to wire racks and let cool completely. Refrigerate at least 2 hours and up to 12 (this will help create clean cuts).

Use a 1 1/2-inch heart-shaped cutter to remove a 3/4-inch-deep heart from each of 6 cupcakes (removing a bit of additional cake, if desired, to create more room for frosting). Fill a pastry bag fitted with a large, plain round tip (such as Ateco #806) with frosting. Pipe frosting to fill heart cutouts. Pipe 1/4 cup frosting on remaining cupcakes. Top frosted cupcakes with heart cutouts. Cupcakes can be stored in an airtight container at room temperature up to 2 days.