Brown butter in a light-coloured saucepan over medium heat, swirling every 30 seconds, until golden-amber with brown specks — 5 to 6 minutes. Pour into a large bowl and cool 15 minutes.
Whisk both sugars into the cooled brown butter until smooth and fully combined.
Add eggs and vanilla. Whisk vigorously for 60 full seconds until the mixture looks glossy and slightly thickened.
Sift in flour, baking soda, cinnamon, and salt. Fold with a spatula until no dry streaks remain.
Fold in rolled oats and mix-ins until evenly distributed. Cover and refrigerate for 30 minutes minimum.
Preheat oven to 175°C (350°F). Line two trays with parchment. Scoop 2-tablespoon balls, spacing 5cm apart.
Bake on middle rack for 11 to 12 minutes until edges are set and golden but centres still look slightly underdone. Rest on tray 5 minutes, then transfer to wire rack.