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.
The dough should be easy to work with and Play-Doh like consistency.If it is very stiff, add a little water, if it is too soft add in a little flour until you get Play-Doh like consistency.
Roll the dough out on a floured surface. Roll to about 1/4-1/2 inch thick.
Use a cookie cutter to cut the cookies out.
Transfer the cookies to a parchment paper lined cookie sheet.
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, if desired and serve. Will keep in an airtight container at room temperature for up to 3 days.