In a small saucepan set over medium heat, brown the butter. Stir the butter occasionally until it turns amber in color and develops little brown specks. At this point, weigh your brown butter to make sure you have at least 90g. If you have a little less, mix in some water until you reach 90g.
Transfer the brown butter to a medium sized mixing bowl and set it in the freezer for 5 minutes to chill. In the meantime, chop up your chocolate bar if using one.
Once the butter has cooled, whisk in both sugars.
Whisk in the egg and vanilla until well combined.
Add in the flour, oats, baking soda, baking powder, and salt. Use a rubber spatula to fold the mixture together until a few flour streaks remain.
Add in the chopped chocolate and fold just until no flour streaks remain.
Cover the bowl with plastic wrap, and chill the dough in the fridge for at least 30 minutes, and up to 48 hours.*
Preheat the oven to 350°F / 175°C. Line a baking sheet with parchment paper or with a silicone mat.
Scoop out heaping 3 tbsp. scoops of dough, setting them on the cookie sheet at least 2" apart to allow room for spreading.
Top each cookie with additional chocolate (optional - but recommended!)
Bake for 12-14 minutes, until the edges of the cookies turn golden brown.
Immediately after taking the cookies out of the oven, ake a bowl that is slightly larger than the cookies and swirl it around over them to make them a little more uniform and circular.
Allow the cookies to rest on the hot baking sheet for at least 5 minutes before transferring to a wire rack to cool. Enjoy!