Espresso Dark Brownie Cupcakes, Peanut Butter Fudge Frosting

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.