In a medium saucepan over medium heat, melt the unsalted butter. Stir frequently until it turns golden brown and emits a nutty aroma, about 5–7 minutes.

Remove the saucepan from heat and allow the brown butter to cool for a few minutes.

In a large mixing bowl, combine the packed brown sugar, granulated sugar, and cooled brown butter. Stir until well combined.

Stir in the large egg and vanilla extract until fully incorporated.

In another bowl, whisk together the all-purpose flour, baking soda, and salt. Gradually add this dry mixture to the wet ingredients, mixing just until combined.

Gently fold in the semi-sweet chocolate chips.

Preheat your oven to 350°F (175°C).

Grease a 10-inch cast-iron skillet or an oven-safe skillet.

Pour the cookie dough into the prepared skillet and spread it out evenly.

Bake in the preheated oven for about 20–25 minutes.

Allow the skillet cookie to cool for about 10 minutes before serving.

Slice into wedges or scoop directly from the skillet.