Combine Dry Ingredients. In a medium mixing bowl, combine gluten-free flour, Dutch processed cocoa powder, cornstarch, baking soda, and salt. Whisk to combine until well blended and set aside.
Cream Butter & Sugars. In the bowl of a stand mixer (or a large bowl) combine 1 cup softened butter with the brown sugar and granulated sugar. Cream on medium speed with the paddle attachment or a hand mixer 2-3 minutes, or until smooth and fluffy.
Finish The Wet Ingredients. Add eggs, one at a time, beating to incorporate each egg fully before adding the next. Scrape down the sides of the bowl and add corn syrup and 1 Tablespoon vanilla. Mix to combine.
Add Dry Ingredients Gradually. Working a bit at a time, mix the dry ingredients 1/3 or 1/2 at a time on low speed, until all of the dry ingredients are just incorporated. (Doing this gradually helps prevent them from poofing out the sides of the mixer!) The sticky batter will look more like very thick brownie batter than traditional cookie dough--this is normal!
Scoop. Working with just 6 cookies at a time, use a large cookie scoop to measure 3-Tablespoon scoops of cookie dough onto a prepared baking sheet. (Don't roll the cookie dough balls smooth, just scoop them straight onto the baking sheet.) Bake & Cool. Place the pan in the oven and bake cookies 9-10 minutes, or until set on top and slightly underdone in the center. (They'll continue to set as they cool.) Allow the cookies to cool 1-2 minutes on the cookie sheet before transferring to a wire rack to cool completely. Repeat process until all cookies are baked, alternating baking sheets with each batch so you don't put cool dough on a hot cookie sheet.