Brown the butter and measure to exactly 3/4 cup, ensuring it's cooled before adding to 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 slightly thickened.

Fold the dry ingredients into the wet mixture using a rubber spatula, stopping once the flour is fully absorbed. If using mix-ins, fold those in before fully absorbed.

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

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

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

Bake large cookies for 12-14 minutes and regular cookies for 10-12 minutes, until golden brown around the edges and puffy in the center.

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

Let cool for about 15 minutes before serving.