Preheat the oven to 350 degrees F. Line standard cupcake pans with 24 cupcake liners.
Combine the butter and chocolate in a small microwave-safe bowl. Microwave on high, uncovered, stirring every 30 seconds, until the butter and chocolate are melted and smooth. Set aside to cool. Mix the flour, cocoa powder, espresso, and salt in another bowl. In the bowl of an electric mixer fitted with the paddle attachment, mix both sugars and reserved chocolate mixture until well combined. Add the eggs, 1 at a time, beating until each is smooth, scraping down the sides of the bowl if needed. Add the flour mixture until blended. Fill the cupcake liners 2/3 full. Bake until golden, rotating the pans halfway though, 18 to 20 minutes. Transfer the pans to wire racks to cool completely before removing the cupcakes.
To assemble: Frost the cooled cupcakes with Peanut Butter Fudge Frosting.
Peanut Butter Fudge Frosting:
In the bowl of an electric mixer fitted with the paddle attachment, beat the butter, peanut butter, and salt on medium-high speed until creamy. Turn the speed to low and beat in the confectioners' sugar, 1/2 cup at a time. Add the cocoa powder and beat until extremely smooth and creamy.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.