Preheat oven to 350°F. Grease or line a 9x9-inch square baking pan with parchment paper.
In a small saucepan over low heat, melt the chopped dark chocolate and butter together, stirring until smooth. Remove from heat and let cool slightly.
½ cup dark chocolate, ¾ cup unsalted butter
Whisk the eggs and sugar. In a large mixing bowl, beat the eggs and brown sugar until pale and slightly thickened.
3 large eggs, 1¼ cups light brown sugar
Combine wet ingredients by stirring in the melted chocolate mixture and vanilla extract until well incorporated.
2 teaspoons vanilla extract
Add the dry ingredients by gently folding in the flour, cocoa powder, and chocolate chips until just combined—do not overmix.
1 cup all-purpose flour, ½ cup unsweetened cocoa powder, ¾ cup semisweet chocolate chips
Pour the brownie batter into the prepared pan, spreading it evenly.
Melt the peanut butter in a microwave-safe bow.
¾ cup creamy peanut butter, ¾ cup unsalted butter
Add the peanut butter layer by drizzling the melted peanut butter over the top and use a knife or skewer to create swirls without fully mixing it in.
Bake in the preheated oven for 25-27 minutes. The brownies should be set with slightly soft centers. A toothpick inserted should come out with moist crumbs.
Cool completely. Let the brownies cool in the pan before using a sharp knife to slice into clean, even squares.
Notes
For clean slices, use a sharp warm knife and wipe it clean between cuts for nice, clean cuts.
Create the perfect swirl by drizzling the melted peanut butter in thin lines over the batter, then using a butter knife to drag through in a zigzag pattern gently—avoid overmixing for defined swirls.