6 Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.
0 In a large mixing bowl, combine the softened butter, brown sugar, and granulated sugar. Use an electric mixer to cream them together until light and fluffy, about 2-3 minutes.
1 Add the egg, egg yolk, and vanilla extract to the bowl. Beat until fully combined and smooth.
4 Lightly pulse the oats in a food processor about 10 times. You want them to have a bit of texture, so avoid grinding them too finely. Set them aside.
3 Gradually add the dry ingredients to the wet mixture, mixing on low speed until just combined. Avoid overmixing (oats, flour, cinnamon, cornstarch, baking soda, baking powder, and salt.)
5 If you want to minimize spreading, refrigerate the dough for at least 30 minutes. If you don’t mind the cookies spreading a bit, you can skip this step.
7 Scoop about 1.5 tablespoons of dough and roll into balls. Place them on the prepared baking sheets, spacing them about 2 inches apart.
8 Bake the cookies for 10-11 minutes, or until the edges are just set and the centers look slightly underbaked. Remember, the cookies will continue to cook on the hot baking sheet as they cool.
9 Immediately after removing the cookies from the oven, use a large glass or round cookie cutter to gently shape them into perfect circles while they’re still soft.
10 Let the cookies cool on the baking sheet for 5 minutes, then transfer them to a wire rack to cool completely.
11 While the cookies are cooling, whisk together the powdered sugar, milk, and a splash of vanilla extract in a small bowl until smooth. If the icing is too thick, add a little more milk (1 teaspoon at a time) until you get your desired drizzling consistency.
12 Dip the top of your cookies lightly in the icing.
