Melt the butter in a 2-quart / 1.9 L saucepan over medium heat, swirling often. Cook for 5 to 6 minutes until it turns amber with small brown flecks on the bottom and smells nutty.

Pour the browned butter into a large heatproof bowl and let it cool for 15 minutes before mixing anything into it.

Whisk the brown sugar and granulated sugar into the cooled browned butter until glossy and fully combined.

Add the eggs one at a time, whisking well after each, then whisk in the vanilla extract until the mixture lightens in color.

In a separate bowl, whisk together the flour, baking soda, salt, and cinnamon.

Fold the dry ingredients into the butter mixture with a spatula just until no dry streaks remain.

Fold in the oats and chocolate chips until they're evenly distributed through the dough.

Cover the bowl and chill the dough for at least 30 minutes, up to 24 hours. Browned butter is more liquid than softened butter, so this step keeps the cookies from spreading thin.

Heat the oven to 175 C / 350 F and line a 33 x 46 cm / 13 x 18 inch sheet pan with parchment.

Scoop the dough into 48 g balls (about 2 tablespoons each) and space them 5 cm apart, roughly 12 to 14 per pan. Bake in two batches so the pan isn't crowded.

Bake for 11 to 13 minutes until the edges are deep golden brown and the centers still look slightly underbaked.

Cool the cookies on the pan for 5 minutes, then transfer to a wire rack to finish cooling completely.