Step 1

Preheat oven to 350°F and line a standard 12-cup muffin tin with paper liners.

Step 2

In a medium bowl, whisk together the flour, cocoa powder, baking soda, baking powder, and salt. In a large bowl, whisk together the sugar, beet purée, buttermilk, butter, chocolate, and eggs. Fold the dry ingredients into the wet.

Step 3

Divide the batter evenly between prepared cups. Bake until the tops are puffed and a toothpick inserted into the center comes out with moist crumbs, 22 to 25 minutes. Transfer cupcakes to a rack to cool completely. Frost as desired.

Step 4

In a large bowl, whisk together the melted chocolate and the sugar and salt until smooth. Stir in the sour cream. Use immediately.