Before You Begin
Allow the browned butter to cool completely before mixing. Line baking sheets with parchment paper. Baking times and results may vary depending on oven accuracy and cookie size.
Brown butter until golden and nutty. Cool completely.
Mix butter with sugars. Add egg, yolk, and vanilla.
Whisk dry ingredients separately.
Combine wet and dry gently. Fold in oats.
Chill dough 30–60 minutes.
Bake at 350°F for 11–12 minutes, or until the edges look set and the centers appear slightly soft. Cookies will firm as they cool.
Cool briefly and serve.