Preheat your oven to 350°F (175°C). Line a 9×9-inch pan with parchment, leaving an overhang to lift the bars later. Lightly grease the parchment so the edges don’t snag.

In a medium saucepan over low heat, melt the butter. Remove from heat and stir in the granulated sugar until glossy.

Whisk in the eggs one at a time so they incorporate smoothly. Add the vanilla and mix until the batter shines.

In a separate bowl, whisk together flour, cocoa powder, salt, and baking powder.

Add the dry mix to the butter-sugar-egg mixture and fold until just combined. Fold in the chocolate chips if using.

Pour the batter into your prepared pan and smooth the top. Bake for 22–28 minutes, until a toothpick comes out with a few moist crumbs.

In a bowl, beat together the peanut butter, powdered sugar, softened butter, and salt until smooth.

Once the brownie base has cooled to room temperature, gently spread the peanut butter layer on top.

Melt the chocolate chips with 2 tablespoons of peanut butter in 20-second bursts, stirring in between. Drizzle or dot over the peanut butter layer and use a knife to create swirls.

Chill the bars for at least 1 hour in the fridge to set the layers, then lift the whole slab out with the parchment and slice into squares.