
Brown the butter: Place butter in a medium-sized saucepan over medium heat. Once melted, increase heat slightly to just above medium. Use a spatula or wooden spoon to constantly stir the butter, scraping the sides and bottom of the pan with the spatula to prevent burning. The butter will sputter and pop for a bit before this slows and it begins to brown (continue stirring during this time), continue stirring. Once you notice a fair bit of browning on the bottom of the pan and the popping has ceased, remove from heat and pour into a large heat-proof bowl. Allow to cool completely to room temperature before proceeding.
Once butter is cooled completely, add sugars, sourdough discard, egg yolks, and vanilla and stir until thoroughly combined.
In a separate medium-sized bowl, whisk together flour, cinnamon, baking soda, and salt.
Gradually add dry ingredients into the wet, stirring until completely combined.
Add oats and stir until combined.
Cover bowl with plastic wrap and chill cookie dough in the refrigerator for at least 30 minutes.
When dough has chilled and you’re about ready to bake, preheat oven to 350F (175C) and line baking sheets with parchment paper.
Once oven is preheated, remove dough from fridge, uncover, and scoop dough by 1 ½ Tablespoon-sized scoop (34g) and drop onto prepared baking sheet, placing at least 2” (5cm) apart.
Transfer to center rack of 350F (175C) and bake for 10-11 minutes, cookies are done when the edges are just beginning to turn a slightly darker brown (they may look slightly underdone at the very centers). Allow cookies to cool completely on baking sheet before enjoying!