Preheat oven to 350F and line three baking trays with parchment paper.

Melt unsalted butter in a saucepan over medium heat for 3 to 5 minutes until browned bits form, then pour into a bowl and let cool for 5 minutes.

Chop the dark chocolate block into small pieces, saving the fine dust.

Whisk the flour, oats, baking soda, and salt together in a large bowl.

Whisk the brown sugar into the cooled browned butter until combined.

Whisk the egg and vanilla extract into the butter mixture until completely smooth.

Add the dry ingredients to the wet mixture and mix with a wooden spoon until mostly incorporated.

Stir in the chopped chocolate and dust until no dry flour remains.

Let the dough rest for 5 minutes to firm up slightly.

Scoop heaped tablespoons of dough onto the prepared trays using a cookie scoop, spacing them apart.

Flatten the dough mounds gently to about a third of an inch thick.

Bake for 10 to 11 minutes, rotating the trays halfway through, until edges are light golden.

Let the cookies cool on the baking trays for 10 minutes before transferring to a wire rack.