Melt the butter in a non-stick pan on medium-low heat until it turns brown with a nutty aroma.
Reduce heat to the lowest setting, add sugars, and whisk for 1-2 minutes to dissolve.
Remove from heat, cool for 5-10 minutes. It should be warm but not hot.
Add egg and yolk, whisk until the mixture is light and smooth (1-2 minutes).
Add dry ingredients, fold with a spatula until almost mixed.
Fold in chopped chocolate until evenly dispersed.
Use a 2" scoop to make 8 ~80g balls. Don't press them, just gently shape.
Optional: Place a few chocolate chunks on top.
Refrigerate for at least 30 minutes, ideally 2 hours or longer.
Preheat oven to 375°F, line a large cookie sheet with parchment paper.
Place balls on the sheet, keeping 3" between each.
Bake for 10-12 minutes, depending on desired softness.