Instructions

• Position an oven rack in the lower-middle position and preheat the oven to 350°F (325°F if using a nonstick muffin pan). Line a standard-size muffin pan with paper liners.
• Combine the 8 tablespoons unsalted butter (cut into 4 pieces), 2 ounces finely chopped bittersweet chocolate, and ½ cup Dutch-processed cocoa powder in a medium heatproof bowl. Set the bowl over a saucepan of barely simmering water and heat, stirring, until the butter and chocolate are melted and the mixture is smooth. (Alternatively, microwave at 50% power, stirring every 30 seconds, until melted.) Set aside to cool until just warm to the touch.
• In a small bowl, whisk together ¾ cup all-purpose flour, ½ teaspoon baking soda, and ¾ teaspoon baking powder. Set aside.
• In a medium bowl, whisk the 2 eggs. Add ¾ cup granulated sugar, 1 teaspoon vanilla extract, and ½ teaspoon table salt and whisk until combined and slightly thickened.
• Whisk the cooled chocolate mixture into the egg mixture until fully incorporated.
• Sift about one-third of the flour mixture over the chocolate mixture and whisk until combined. Whisk in the ½ cup sour cream until combined. Sift the remaining flour mixture over the batter and whisk until the batter is homogeneous and thick.
• Divide the batter evenly among the prepared muffin cups (about 12 standard cupcakes).
• Bake until a skewer inserted into the center comes out clean, 18 to 20 minutes. Remove from oven.
• Cool the cupcakes in the muffin pan on a wire rack for 15 minutes, then transfer the cupcakes to the wire rack and cool to room temperature (about 30 minutes) before frosting.
• Make the peanut butter frosting: in the bowl of an electric mixer fitted with the paddle attachment, combine 1 cup powdered sugar, 1 cup creamy peanut butter, 5 tablespoons unsalted butter (at room temperature), ¾ teaspoon vanilla extract, and ¼ teaspoon kosher salt. Mix on medium-low speed until creamy, stopping to scrape down the bowl as needed.
• Add ⅓ cup heavy cream to the frosting and beat on high speed until the frosting is light and smooth.
• Frost the cooled cupcakes using an offset spatula or a piping bag and tip. Store frosted cupcakes at room temperature in an airtight container for up to 4 days.