Preheat oven to 350F (175C) and line a cookie sheet with parchment paper. Set aside.
Combine melted, cooled butter and sugars and stir until well-combined.
1 cup unsalted butter,1 cup dark brown sugar,½ cup granulated sugar
Add egg yolks and vanilla extract and stir well. Set aside.
2 large egg yolks,1 ½ teaspoons vanilla extract
In a separate, medium-sized bowl, whisk together flour, baking soda, baking powder, salt, and cinnamon.
2 cup all-purpose flour,1 teaspoon baking soda,1 teaspoon baking powder,1 teaspoon salt,½ teaspoon cinnamon
Gradually stir dry ingredients into wet until completely combined.
Now you’ll need to prepare your oats. Add old fashioned oats to the basin of a food processor and pulse briefly (7-10 times, don’t over-do it or you’ll end up with oat flour!)
2 cups old fashioned rolled oats
Stir your oats into your cookie dough until well incorporated.
Scoop dough by heaping 2 teaspoon-sized scoops and gently roll between your palms to form a smooth round ball (dough may be crumbly). Place cookie dough at least 2″ apart on baking sheet and use your fingers to gently press down on each cookie to lightly flatten.
Bake on 350F (175C) for 12 minutes.
Allow cookies to cool completely before covering with icing.