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.