In a large mixing bowl or stand mixer, cream together the softened butter, granulated sugar, and brown sugar on medium-high speed until light and fluffy (2-3 minutes).

Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract until fully incorporated.

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

Gradually add the dry ingredients to the wet ingredients, mixing on low speed until a soft dough forms.

Divide the dough into two equal portions.

To one portion of the dough, add cocoa powder and milk. Mix on low speed until fully integrated.

Fold half of the chocolate chips into the brownie dough, and fold the remaining half into the vanilla dough.

Take a tablespoon-sized piece of both the vanilla and brownie doughs, gently press them together and roll into a ball.

Twist lightly as you roll to create a marble swirl effect.

Place the balls about 2 inches apart on a lined baking sheet.

Preheat your oven to 350°F (175°C).

Bake for 10 to 12 minutes until the edges are set and lightly golden, while the centers remain soft.

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