Prepare the Brownie Cookie Dough:

Preheat oven to 350°F (175°C). Line baking sheets with parchment paper. In a microwave-safe bowl, melt butter and chocolate chips in 30-second intervals until smooth. Cool slightly before adding sugar, eggs, and vanilla. Mix dry ingredients (flour, cocoa powder, baking powder, salt) separately, then fold them into the chocolate mixture. Set aside.

Prepare the Peanut Butter Cookie Dough:

Beat softened butter and peanut butter until smooth. Add sugars and beat until fluffy. Add egg and vanilla, then mix in dry ingredients (flour, baking powder, baking soda, salt).

Swirl and Bake:

Scoop a tablespoon of both brownie dough and peanut butter dough onto the baking sheet, leaving 2 inches between each. Swirl the two doughs with a butter knife or small spatula. Optionally roll the doughs into balls and press lightly. Bake for 10-12 minutes until the edges are set but centers are soft. Let cool for 5 minutes on the baking sheet before transferring to a wire rack to cool completely.