Preheat oven to 350°F (175°C) and line two large baking sheets with parchment paper.
In a medium bowl, cream together the softened butter, brown sugar, and 1/4 cup granulated sugar until light and fluffy.
Beat in one egg and 1 teaspoon vanilla extract for the cookie dough.
Stir in 1 1/2 cups flour, baking soda, and 1/2 teaspoon salt until combined, then fold in chocolate chips; set cookie dough aside.
In a separate bowl, whisk together the melted butter and 1 cup granulated sugar for the brownie batter.
Whisk in the remaining egg and 1 teaspoon vanilla extract.
Sift in the cocoa powder, 1/2 cup flour, and 1/4 teaspoon salt, stirring until just combined.
Scoop one tablespoon of cookie dough and one tablespoon of brownie batter, gently swirling them together by hand to create a marbled effect.
Place the swirled dough balls onto the prepared baking sheets, spaced 2 inches apart.
Bake for 10 to 12 minutes until the edges are set but the centers remain slightly soft.
Allow cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.