Preheat the oven to 350°F (175°C) and line a baking pan with parchment paper.

In a saucepan, brown the butter over medium heat, stirring frequently until golden brown and nutty. Remove from heat and let cool slightly.

In a mixing bowl, combine the browned butter with both sugars and mix until well combined.

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

In another bowl, whisk together the flour, baking soda, and salt, then gradually add to the wet mixture until just combined.

Fold in the chocolate chips (and nuts or marshmallows, if using).

Spread the dough in the prepared pan, creating swirls of brownie and cookie dough.

Bake for 25-30 minutes or until the edges are set and the center still jiggles slightly.

Cool before cutting.