Iced Oatmeal Cookies

Name: Iced Oatmeal Cookies
Prep time: 10 min
Cook time: 10 min
Total time: 20 min
Cuisine: American
Ingredients: 12 Tbsp. butter, unsalted; 1/2 cup brown sugar, tightly packed (light or dark); 1/3 cup white sugar; 1 large egg, room temperature; 1 large egg yolk, room temperature; 1 tsp. vanilla extract; 1 cup old fashioned oats; 3/4 cup all-purpose flour, spooned and leveled; 3/4 cup cake flour, spooned and leveled; 1 tsp. cornstarch; 1/2 tsp. baking soda; 1/2 tsp. baking powder; 1/2 tsp. salt; 1 tsp. ground cinnamon; 1/2 tsp. ground nutmeg; 1 cup powdered sugar, or more as needed; 2 Tbsp. milk; 1/2 tsp. vanilla extract; pinch of salt