Preheat oven to 350°.
In the bowl of a stand mixer, cream the butter and sugars together for 2-3 minutes or until light (in color!) and fluffy.
Scrape the sides of the bowl and then add in the eggs and vanilla. Mix until combined.
Add the flour, cocoa, soda, salt and cornstarch and mix together. Start mixing slowly, gaining speed as the flour incorporates. Mix until the dough has come together.
Scoop the dough and arrange it onto a cookie sheet.
Use the back of a drinking glass to flatten the cookies.
Bake at 350° for 6-8 minutes or until the cookies are puffy and no longer glossy.
Let the cookies rest on the hot cookie sheet for 5 minutes before transferring the a metal cooling rack.
Frost the cookies with Vanilla Buttercream Frosting and sprinkle the tops of the cookies with the crushed peppermint candies.