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

Dump in the dry ingredients and fold together using a rubber spatula until flour is fully absorbed; if using mix-ins, fold them in just before the flour is fully absorbed.

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

Preheat the oven to 350?F and line a large baking sheet with parchment paper when the 20 minutes is almost up.

Roll the dough into balls and place them a few inches apart on the baking sheet; gently press them down to about 1/2 inch thickness.

Bake large cookies for 12-14 minutes and regular cookies for 10-12 minutes; they are done when 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 them cool for about 15 minutes before serving.