Preheat oven to 350°F. Line two cookie sheets with a silicone baking mat.
Using a standing mixer with paddles attached, cream the butter, sugar, light brown sugar, eggs, corn syrup, and vanilla until light and fluffy, about 5-7 minutes.
In a medium bowl, whisk the flour, cocoa powder, cornstarch, salt, and baking soda.
Gradually beat the dry ingredients into the wet until a dough forms.
Using a medium cookie scoop, scoop out dough mound into your hand and roll into a ball. Place onto the cookie sheet and light press into a thick disk.
Bake in the preheated oven for 15-18 minutes or until firm around the edge.
Allow to cool for a few minutes on the baking sheet. Then move the cookies to a cooling rack to cool completelly.