Brown the butter and measure it to exactly 3/4 cup, ensuring it is no longer warm before adding to the dough.

In a medium bowl, whisk together the flour, oats, baking soda, and salt; set aside.

In a large bowl, whisk together the cooled brown butter, brown sugar, eggs, and vanilla until smooth and thickened to a caramel color.

Fold the dry ingredients into the wet mixture using a rubber spatula until the flour is fully absorbed; if using mix-ins, fold them in before fully incorporating the flour.

Place the dough uncovered in the fridge for about 20 minutes.

Preheat the oven to 350F and line a baking sheet with parchment paper.

Scoop the dough into balls (2oz each for large cookies or smaller for regular cookies) and place them a few inches apart on the baking sheet, pressing down to about 1/2 inch thickness.

Bake large cookies for 12-14 minutes and regular cookies for 10-12 minutes until there is a golden brown ring around the edges and the center is pale and puffed.

Sprinkle with flaky sea salt and transfer to a cooling rack.

Let cool for about 15 minutes before serving.