Preheat the oven to 375ºF.

In an 11” skillet*, brown the butter over medium heat. Stir constantly and scrape the bottom of the pan with a heatproof rubber spatula. Once the butter melts it will begin to bubble before developing a layer of foam on top. When the particles at the bottom of the pan start to brown and develop an amber color, remove from the heat. 

Allow the browned butter to cool in the skillet for 15 to 20 minutes. While the butter is cooling, begin preparing the remaining ingredients.

Add the granulated sugar, and brown sugar to the skillet with the cooled browned butter and stir until combined.

Add the whole egg and two egg yolks and stir until fully incorporated.

Add the vanilla extract and stir together.

In a separate large bowl, mix together the all-purpose flour, rolled oats, baking soda, baking powder, kosher salt, and cinnamon.

Add the dry ingredients to the skillet and fold into the wet ingredients until just combined.

Fold in the chopped chocolate and chopped walnuts until evenly distributed throughout the dough.

Spread the cookie dough evenly across the skillet, before placing in the oven. 

Bake at 375ºF for 25 minutes, until the top is golden brown