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.
Add egg yolks and vanilla extract and stir well. Set aside.
In a separate, medium-sized bowl, whisk together flour, baking soda, baking powder, salt, and 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!)
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.
TO MAKE ICING
Combine powdered sugar, 1 ½ Tablespoons milk, corn syrup, and vanilla extract in a small bowl. Whisk until well-combined, if frosting is too thick, add more milk as needed. The frosting has reached the correct consistency when you lift up the spoon and it drizzles a thin ribbon of icing back into the bowl, that should hold its shape for just a second before dissolving back into the rest of the icing (see my video below for a visual).
To dip cookies, grip cookie by the base and dip just the surface of each cookie into the frosting. Pull straight out and place dipped cookie icing-side-up on a cooling rack to set. It will take several hours at room temperature for the icing to set completely and for the cookies to be stackable.
*When measuring flour using cups, never scoop the flour directly into the measuring cup (this is likely to give you more flour than you need, resulting in dry cookies). Instead, stir your flour in its container then spoon the flour into your measuring cup and level it off using the back of a knife. **While my preference is to use corn syrup (which is not the same as HFCS!) You can get away with making this without the corn syrup, the frosting will be less shiny and less firm when set, but it will work.