Instructions

• Preheat the oven to 350°F (180°C). Line two 12-cup muffin pans with cupcake liners.
• In a medium bowl, whisk the Dutch-process cocoa powder and very hot coffee together until it forms a thick, smooth paste.
• In a separate bowl, whisk together the buttermilk, vegetable oil, 1 teaspoon vanilla extract, 1 large egg, 1 large egg yolk, and 1/2 teaspoon salt. Gradually stir the buttermilk mixture into the cocoa paste until smooth and uniform.
• Sift together the all-purpose flour, 1 3/4 cups sugar, baking powder, and baking soda into a large bowl. Stir the dry ingredients into the cocoa mixture just until combined; do not overmix.
• Divide the batter among the prepared muffin cups, filling each about two‑thirds full. (Use a measuring cup or a spring-loaded scoop for even portions.)
• Bake the cupcakes 20–25 minutes, until a toothpick inserted in the center comes out clean or the centers spring back lightly when touched. Cool the cupcakes completely on a wire rack before frosting.
• While the cupcakes cool, make the peanut butter buttercream. Combine 1 cup water, 2 cups sugar, 2 tablespoons light corn syrup, and 1/4 teaspoon cream of tartar in a heavy, nonreactive saucepan fitted with a candy thermometer. Heat over medium-high, stirring to dissolve the sugar, and bring the syrup to 235°F (113°C).
• Meanwhile, place the 5 egg yolks and 1 whole egg in the bowl of a stand mixer fitted with the whisk attachment. Whip on high until the eggs are thick and hold their shape.
• When the syrup reaches 235°F, briefly stop the mixer, add the 1/8 teaspoon xanthan gum to the whipped eggs, then restart the mixer. With the mixer on medium-high, carefully and slowly pour the hot syrup in a thin, steady stream into the eggs, aiming between the whisk and the side of the bowl. Pour gradually to avoid cooking the eggs.
• After all the syrup is incorporated, increase the mixer speed to high and whip until the outside of the mixing bowl feels cool to the touch.
• Switch to the paddle attachment. Cut the cold butter into small slivers. With the mixer on medium-high, add the butter pieces one at a time, waiting until each piece is mostly incorporated before adding the next. Continue until all 1 1/2 pounds (680 g) butter has been added and the mixture is smooth. If the buttercream looks separated at any point, continue beating and it should come together.
• Reduce the mixer speed to low and add 3/4 cup (180 g) natural peanut butter (plus additional peanut butter if you want a stronger flavor), 1 teaspoon kosher or sea salt, and 1/2 teaspoon vanilla extract. Taste and, if desired, add more peanut butter a tablespoon at a time and/or adjust salt to taste.
• Frost the completely cooled cupcakes. You may spread the buttercream thickly with a spatula or pipe mounds using a pastry bag fitted with a large plain tip (about 1/2-inch / 1.75 cm). Refrigerate the frosted cupcakes for 20 minutes to firm the frosting.
• Make the peanut-pretzel topping: pulse 3/4 cup (110 g) roasted peanuts and 1 1/2 cups (90 g) broken-up large pretzels in a food processor until you have a mix of fine crumbs and some larger pieces, or crush in a zip-top bag with a rolling pin. Transfer the mixture to a bowl.
• Press the tops of the chilled, frosted cupcakes into the peanut-pretzel mixture so the pieces adhere. If the buttercream is too firm for the topping to stick, let the frosted cupcakes sit at room temperature a few minutes, then press on the topping.
• Make the ganache: warm 1/4 cup (60 ml) heavy cream in a small saucepan until it is just starting to simmer. Remove from heat, add 2 ounces (55 g) chopped bittersweet or semisweet chocolate, and let sit 2 minutes. Stir until smooth. If the ganache is too thick to drizzle, stir in a little additional warmed cream, a small amount at a time, until it reaches a drizzling consistency.
• Use a teaspoon to drizzle the ganache over each cupcake. Finish each cupcake with a light pinch of kosher or sea salt, if desired.