Preheat the oven to 350°F. Line a baking sheet with parchment paper.

Cream the butter, sugar, and brown sugar in a large bowl until light and fluffy.

Add the egg and vanilla extract; mix until well combined.

Whisk together the flour, baking powder, baking soda, and salt in a separate bowl.

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

Fold in the chocolate chips.

Divide the dough in half. To one half, stir the cocoa powder to create the chocolate dough.

Take small spoonfuls of the vanilla cookie dough and the chocolate cookie dough and gently press them together, swirling slightly without thoroughly mixing, to form marbled cookie balls.  You can use your cookie scoop to create the dough balls.

Place the dough on a prepared baking sheet, leaving space between the cookies.

Bake for 9–11 minutes or until the edges are set and the centers look slightly underbaked.

Cool on the baking sheet for 5 minutes before transferring to a wire rack.