Prepare the Peanut Butter Cookie Dough:

In a large bowl, cream together the softened butter, peanut butter, granulated sugar, and brown sugar until light and fluffy.

Beat in the egg and vanilla extract until well combined.

In a separate bowl, whisk together the all-purpose flour, baking soda, and salt.

Gradually add the dry ingredients to the wet mixture, mixing until just combined.

Cover the dough and refrigerate for at least 30 minutes.

Prepare the Brownie Cookie Dough:

In a microwave-safe bowl, melt the butter and let it cool slightly.

Stir in the granulated sugar until combined.

Add the eggs and vanilla extract, mixing well.

Sift in the unsweetened cocoa powder, all-purpose flour, baking powder, and salt; stir until just combined.

Fold in the semisweet chocolate chips.

Cover the dough and refrigerate for at least 30 minutes.

Preheat the Oven:

Preheat your oven to 350°F (175°C).

Line baking sheets with parchment paper.

Form the Cookies:

Scoop 1 tablespoon of peanut butter cookie dough and 1 tablespoon of brownie cookie dough.

Gently press the two doughs together and roll into a ball, creating a marbled effect.

Place the dough balls onto the prepared baking sheets, spacing them about 2 inches apart.

Bake:

Bake in the preheated oven for 10-12 minutes, or until the edges are set and the centers are still slightly soft.

Allow the cookies to cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely.