Preheat the oven to 175ºC conventional oven. Prep a 20x20 cm square pan with parchment paper.
In a bowl add the butter, peanut butter, brown sugar and granulated sugar, and with an electric hand mixer, whisk it together for 1-2 min.
35 g butter, 30 g smooth peanut butter, 40 g brown sugar, 70 g granulated sugar
Add the egg and mix it in. Then add the vanilla extract and mix it in.
1 large egg, 1 teaspoon vanilla extract
In a separate bowl, stir together all-purpose flour, baking powder, and salt. Then add it into the butter sugar mixture. With a rubber spatula, fold it together until just combined.
125 g all-purpose flour, ¼ teaspoon baking powder, ¼ teaspoon salt
Add ¾ off the cookie dough to the bottom of the baking pan and even it out. Set it aside while making the brownie batter.