To brown the butter: In a large skillet, melt butter over medium heat. When it begins to bubble, slowly begin swirling pan. Continue swirling until bubbles increase and turn into foam and then dissipate, leaving browned bits along the bottom of the pan. Immediately pour into a bowl to cool, taking care to scrape the bottom of the pan for all the delicious browned butter bits.
Whisk together the oats, flours, soda, powder, salt and cinnamon, set aside.
In the bowl of a stand mixer fitted with a paddle attachment, cream the cooled butter with the sugars. Scrape down the sides of bowl as necessary.
Add the eggs one at a time, mixing thoroughly after each addition. Stir in vanilla.
In three batches, add the dry ingredients to the wet. Scrape down sides of bowl to be sure dough is thoroughly combined.
Wrap dough in plastic wrap and refrigerate for two hours or until ready to use.
When ready to bake, preheat oven to 350 degrees. Line baking sheets with parchment paper or silicone mats.
Using a small cookie scoop, portion out balls of dough and place 2 inches apart on prepared baking sheets. The cookie will spread out considerably so leave adequate room.
Bake in preheated oven for 8-10 minutes or until edges are golden brown and center has started to set. Remove to rack to cool.
While cookies are cooling, sprinkle lightly with flaked sea salt.