Instructions

• Preheat oven to 350°F (175°C). Line two regular-size cupcake pans with paper liners and place the pans on oven racks.
• In a large mixing bowl, whisk together 1¾ cups all-purpose flour, ⅔ cup unsweetened cocoa powder, 1½ teaspoons baking powder, and 1½ teaspoons baking soda until evenly combined.
• Add 2 cups granulated sugar and ½ teaspoon table salt to the dry mixture. Whisk to combine.
• Add ¾ cup sour cream, ⅔ cup vegetable oil, and 2 teaspoons pure vanilla extract to the dry ingredients. Whisk until mostly smooth and no large lumps remain.
• Add the 2 large eggs one at a time, whisking after each addition until the yolk and white are fully incorporated and there are no streaks of egg remaining.
• Carefully pour in ¾ cup boiling water and whisk until the batter is smooth. The batter will be thin—this is normal.
• Divide the batter among the lined cupcake wells, filling each about 2/3 full.
• Bake for 22 to 25 minutes, or until a toothpick inserted into the center of a cupcake comes out clean or with a few moist crumbs (not wet batter).
• Remove the pans from the oven and let the cupcakes cool in the pans for about 5 minutes. Then transfer the cupcakes to a wire rack and allow them to cool completely before frosting.
• While the cupcakes cool, make the frosting. In the bowl of a stand mixer or a large mixing bowl with a handheld mixer, beat 1 cup softened salted butter, ⅓ cup unsweetened cocoa powder, and 2 teaspoons pure vanilla extract on medium-high speed for 1½ to 2 minutes, until smooth and creamy.
• With the mixer on low speed, add powdered sugar in portions: add 1½ cups powdered sugar and beat until incorporated.
• Add 2 to 2½ tablespoons heavy cream and beat until combined and smooth.
• Add the next 1½ cups powdered sugar and beat until incorporated. Add another 2 to 2½ tablespoons heavy cream and beat until combined.
• Add the remaining 1 cup powdered sugar and beat on medium speed until the frosting is smooth and spreadable. If needed for consistency, use any remaining portion of the 4 to 5 tablespoons heavy cream (total added should be 4 to 5 tablespoons) and beat until you reach a smooth, pipeable texture. Scrape the bowl as needed.
• Transfer the frosting to a piping bag fitted with a large decorator tip (such as 1M, 2D, or 4B) or to a disposable bag with a corner snipped off.
• Hold the piping tip about ½ inch above a cooled cupcake. Starting at the outer edge, pipe the frosting in an even spiral toward the center (or start in the center and spiral outward, if you prefer).
• If desired, sprinkle chocolate sprinkles over the frosted cupcakes as a garnish.
• Repeat piping and garnishing for the remaining cupcakes. Store finished cupcakes at room temperature for up to 2 days or refrigerate if keeping longer (bring to room temperature before serving).