In a large mixing bowl, cream together ½ cup of softened unsalted butter with ½ cup brown sugar and ½ cup granulated sugar until light and fluffy, about 2-3 minutes. Add two room temperature eggs one at a time, mixing well after each addition, and include 1 teaspoon of vanilla extract.

In a separate bowl, whisk together 1 cup of all-purpose flour, ¼ cup of unsweetened cocoa powder, and 1 teaspoon of baking powder until thoroughly combined. Gradually fold this dry mix into the wet ingredients from Step 1, stirring gently until a cohesive dough forms.

In another bowl, melt ½ cup of butter, and whisk in ½ cup of granulated sugar, 1 room temperature egg, and ¼ cup of all-purpose flour along with ¼ cup of cocoa powder. Stir until smooth to create a fudgy brownie batter.

Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper. Drop dollops of the cookie dough and brownie batter alternately onto the parchment. Swirl the two batters together to create a marbled effect.

Place the baking sheet in the preheated oven and bake for about 12 minutes. The edges should be set but the centers should still be soft.

Allow the cookies to cool on the baking sheet for about 5 minutes, then transfer them to wire racks to cool completely.