Preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper.

In a large mixing bowl, combine the melted butter and granulated sugar. Whisk until well combined.

Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

In a separate small bowl, whisk together the flour, cocoa powder, baking powder, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Do not overmix. Fold in the chocolate chips, if using.

In a medium mixing bowl, cream together the room temperature butter and creamy peanut butter with an electric mixer until light and fluffy (about 2-3 minutes).

Add the granulated sugar and light brown sugar to the peanut butter mixture and beat until well incorporated.

Beat in the egg and vanilla extract until smooth.

In a separate small bowl, whisk together the flour, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Do not overmix.

Scoop about 1 tablespoon of brownie dough and 1 tablespoon of peanut butter dough for each cookie. Gently flatten each scoop slightly.

Place the two flattened doughs next to each other and gently press them together. Lightly swirl them with your fingers to create a marbled effect, without fully combining them. Alternatively, you can layer a smaller piece of peanut butter dough on top of a larger brownie dough piece and swirl.

Place the combined dough balls about 2 inches apart on the prepared baking sheets.

Bake for 10-12 minutes, or until the edges are set and the centers look slightly soft. The brownie parts will still look gooey.

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