For the Dark Chocolate Cupcakes:

1/2 cup unsalted butter, cut into 4 pieces
2 oz bittersweet chocolate, finely chopped
1/2 cup Dutch-processed cocoa
3/4 cup all-purpose Gold Medal flour
1/2 teaspoon baking soda
3/4 teaspoon baking powder
2 large eggs, at room temperature
3/4 cup granulated sugar
1 teaspoon vanilla extract
1/2 teaspoon salt
1/2 cup plain Greek yogurt or sour cream (I used Chobani Greek yogurt)
12 Reese's Peanut Butter Cups

For the Peanut Butter Frosting:

1 cup unsalted butter, at room temperature
1 cup of cream peanut butter
2 1/2 cups powdered sugar
1 tablespoon heavy cream
Mini Peanut Butter Cups, for topping cupcakes, if desired