Brown the butter: In a saucepan over medium heat, melt the butter and continue cooking, stirring often, until it foams and turns golden brown. It will smell nutty. Remove from heat and let cool for 10 minutes.
Mix sugars and butter: In a large bowl, whisk the brown sugar and granulated sugar with the browned butter until smooth.
Add egg, yolk, and vanilla: Stir in the egg, extra yolk, and vanilla extract until fully combined.
Combine dry ingredients: In a separate bowl, whisk together the flour, baking soda, and salt.
Mix it all together: Add the dry ingredients to the wet mixture. Stir until just combined.
Fold in oats and chocolate: Add the rolled oats and chocolate chunks, gently folding until everything is evenly distributed.
Optional chill time: For thicker cookies, chill the dough for 30–60 minutes.
Preheat the oven: Set oven to 350°F (177°C) and line baking sheets with parchment paper.
Scoop and space: Scoop about 2 tablespoons of dough for each cookie and place them 2 inches apart on the prepared trays.
Bake until golden: Bake for 9–11 minutes or until the edges are set and the centers still look a little soft.
Add flaky salt and cool: Sprinkle with flaky salt while still warm. Let cookies cool on the tray for 5–10 minutes before transferring.
Serve and enjoy: Eat them warm or cool—either way, they’re packed with flavor and impossible to resist.