Chop 12 small chocolate-covered
pretzels (1/2 cup chopped). Fold into batter.
To frost: Melt 3 oz coarsely chopped semisweet chocolate in microwave for 1 1/2 minutes. Stir until smooth. Cool slightly. Beat 1/2 cup (1 stick) softened unsalted butter with 2 cups confectioners' sugar and 2 tbsp milk. Beat in 1 heaping tbsp of the melted chocolate. Transfer remaining chocolate to a resealable plastic bag. Spread frosting over cupcakes. Snip a small corner from bag and drizzle chocolate over cupcakes (reheat if needed).
Top with 1/2 cup chopped chocolate-covered pretzels.