Preheat your oven to 350°F. Grease a square 8x8 inch baking dish or line it with parchment paper, leaving some overhang for easy removal.

In a large bowl, whisk together the melted butter and granulated sugar until smooth and glossy.

Whisk in the eggs and additional egg yolk one at a time, ensuring each is fully incorporated before adding the next. Stir in the vanilla extract.

In a separate medium-sized bowl, sift together the cocoa powder, all-purpose flour, salt, baking powder, and instant coffee powder.

Gradually fold the dry ingredients into the wet ingredients until just combined.

Fold in the chocolate chips.

Pour the batter into the prepared pan in an even layer. Dollop small spoonfuls of peanut butter on top of the brownie batter and use a knife to create swirls.

Place in the preheated oven and bake for 35-40 minutes or until a toothpick inserted into the center comes out with a few crumbs. The center should be set but still slightly gooey.

Allow the brownies to cool completely in the baking dish on a wire rack. Once cooled, lift out using the parchment paper overhang and cut into squares.