For the cupcakes:
½ cup plus 1 tablespoon Dutch-process cocoa powder
½ cup plus 1 tablespoon hot water
2¼ cups all-purpose flour
¾ teaspoon baking soda
¾ teaspoon baking powder
½ teaspoon salt
2 sticks plus 1 tablespoon unsalted butter, at room temperature
1⅔ cups sugar
3 large eggs, at room temperature
1 tablespoon vanilla extract
¾ cup sour cream
For the frosting:
¾ cup heavy cream, chilled
3¼ cups confectioners’ sugar, sifted, divided
1½ cups (3 sticks) unsalted butter, at room temperature
1 cup creamy peanut butter
1 tablespoon vanilla extract
Pinch of coarse salt
Melted dark chocolate for drizzling, optional
Heat oven to 350º. Line cupcake pans with paper liners. Combine the cocoa powder and water in a small bowl and whisk until smooth. Combine the flour, baking soda, baking powder and salt in a medium bowl and whisk to combine.
Combine the butter and sugar in a medium saucepan and place over medium heat. Cook, stirring occasionally, until the butter is completely melted and the mixture is smooth. Transfer to the bowl of an electric mixer and beat on medium-low speed for 4-5 minutes or until the mixture is cool. Beat in the eggs one at a time, mixing well after each addition. Blend in the vanilla and cocoa powder and continue mixing until smooth, scraping down the sides of the bowl as needed.
With the mixer on low speed, blend in the flour mixture in three additions, alternating with the sour cream, beginning and ending with the dry ingredients. Mix each addition until just incorporated, then scrape down the sides and bottom of the bowl. Divide the batter evenly between the prepared liners and bake until a toothpick inserted in the center comes out clean, 18-20 minutes, rotating the pans halfway through baking. Cool in the pans for at least 5 minutes, then transfer to a cooling rack to cool completely before frosting.
While the cupcakes are cooling, make the frosting. Combine the heavy cream and ¼ cup of the confectioners’ sugar in the clean bowl of an electric mixer fitted with the whisk attachment. Whip on medium-high speed until stiff peaks form, taking care not to over mix. Transfer the whipped cream to a separate bowl.
In the now empty mixer bowl fitted with the paddle attachment, combine the butter and peanut butter. Beat on medium-high speed until smooth, about 45 seconds. Reduce speed and slowly add the remaining confectioners’ sugar to the bowl and mix in, gradually increasing the speed. Add the vanilla extract and salt, and continue beating on high speed until very fluffy, about 4-5 minutes, scraping down the sides and bottom of the bowl as needed.
Use a large spatula to gently fold about a third of the whipped cream into the peanut butter frosting to lighten it. Once incorporated, gently fold in the remaining whipped cream until no streaks remain.
Frost the cupcakes as desired and drizzle with melted chocolate.