Pre-heat your oven to 350℉ and line your baking sheets with parchment paper.
In a large bowl add your butter and melt. Add your sugar and brown sugar. Stir until well combined.
Add your eggs and vanilla and stir until light and fluffy.
Next, add in your dry ingredients which includes your cornstarch, baking soda, baking powder, salt, flour, and oats. Stir until just combined.
Using a large cookie scoop, scoop your cookie dough into 1-2 inch balls and place on your baking sheets lined with parchment paper a few inches apart to prevent from spreading into each other.
Bake in the oven at 350℉ for 8-10 minutes. Remove from oven and using a round cookie cutter or glass, scoot the edges of your cookies in to make them all round and to help out with spreading.
In a large measuring cup, add 2 cups of powdered sugar and 1/4 cup of milk and whisk until well combined.
Once your cookies have cooled completely, dip tops of them in your icing and then let them set on a cooling rack.
Top with festive sprinkles if you'd like, serve, share and enjoy!!