6 ounces semisweet chocolate chips
8 tablespoons (1 stick, 4 ounces) unsalted butter
2 large eggs
1/2 cup granulated sugar
1/2 cup dark brown sugar
1 tablespoon instant espresso coffee powder
1 teaspoon vanilla extract
2/3 cup all-purpose flour
2/3 cup unsweetened cocoa powder
1/2 teaspoon salt
1/4 teaspoon baking powder
1/2 cup smooth peanut butter
2 tablespoons powdered sugar
Preheat the oven to 350°F. Line an 8-inch square baking dish with parchment paper; set aside.
Set the chocolate and butter in a heat-proof bowl over a pot of simmering water—the bottom of the bowl should not touch the water. Stir until the chocolate has melted. Remove from heat.
Whisk together the eggs, both sugars, coffee powder, and vanilla extract. Add in the flour, cocoa powder, salt, baking powder, and melted butter-chocolate mixture and stir until evenly combined. (The batter will be very thick, but don't worry! That's how it's supposed to be.)
Press the brownie batter into the prepared baking dish in an even layer.
Warm the peanut butter in the microwave until it's loosened enough to drizzle, about 30 seconds on high. Stir the powdered sugar into the peanut butter, then drizzle over the top of the brownie batter.
Use a butter knife to swirl the peanut butter mixture into the brownie batter.
Bake the brownies until a toothpick inserted in the middle comes out mostly clean, about 35 minutes. (The peanut butter will probably stick to the toothpick a little bit; that's fine.) Transfer to a wire rack to cool.
Let the brownies cool completely to room temperature before cutting them. For cleaner slicing with fewer crumbs, pop the pan in the freezer for a few minutes once it's cooled to room temperature to help with slicing to help keep them from sticking to the knife.