In a saucepan, add the butter (that’s been cut into slices) over medium heat. Melt the butter and continue to cook, stirring occasionally, until the butter smells nutty and turns an amber color, about 5-7 minutes. Pour the browned butter into a measuring cup. It won’t quite be 1 cup of browned butter, add a little water to bring it back up to 1 cup. Then pour into the bowl of a stand mixer to cool.
Preheat oven to 350 degrees and line a baking sheet with parchment paper or a silicone mat. In a medium-sized bowl, whisk together the flour, baking soda, salt and espresso powder. Set aside.
After the browned butter has cooled, add both sugars to the bowl. Mix on medium speed for 3-4 minutes until it’s light and fluffy. Turn the speed down and add the vanilla extract and the eggs (one at a time). Mix until fluffy and incorporated, 1-2 minutes. Scrape down the sides of the bowl as necessary.
Turn the mixer off and add half of the flour mixture. Turn the mixer on low and mix until just combined. Turn the mixer off and add the other half of the flour. Mix on low until incorporated and scrape the sides of the bowl.
Remove the bowl from the mixer stand and stir in the old fashioned oats by hand. Then add the chopped chocolate pieces and mix together by hand.
You can bake the cookies immediately, but I’d recommend letting the dough chill for 30 minutes so they won’t spread as much. Keep the dough in the refrigerator in between batches.
To bake them- add 2 tablespoons of dough to your prepared baking sheet. Space the dough 2-3 inches apart, do NOT flatten. Bake for 10-12 minutes until the edges of the cookies are golden brown. If you are using the sea salt, sprinkle some onto the cookies immediately when they come out of the oven.
Let the cookies sit on the baking sheet for 5 minutes until removing to a cooling rack. Enjoy!