Preheat & Prep. To start, preheat the oven to 350 degrees F. and line 2 sheet pans with parchment paper. Set aside.

Combine Dry Ingredients. In a medium 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 mixing bowl) combine 1 cup softened butter with the brown sugar and granulated sugar. Cream on medium speed with the paddle attachment or an electric 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 batter will look more like very thick brownie batter than traditional cookie dough--this is normal!

Scoop & Sprinkle. 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 them into tight balls, just scoop them straight onto the baking sheet.) Sprinkle each cookie dough ball with a few gluten-free Oreo cookie crumbs.

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.) If desired, gently coax the cookies into perfect circles with a large cookie cutter. 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. 

Beat Butter. When the cookies are all cool, make the frosting. To start, beat 1 cup softened butter 2-3 minutes in a stand mixer with a paddle attachment (or a large mixing bowl with an electric mixer). The butter should be creamy and lighter in color. 

Combine. Scrape down the sides of the bowl and add cocoa powder, 3 Tablespoons cream, 1 Tablespoons corn syrup, the vanilla, and a pinch of salt. Mix on LOW speed until just blended. Add the powdered sugar gradually, along with 2 Tablespoons more cream.

Whip. As soon as the powdered sugar is all added, increase the speed to HIGH and mix 1-2 minutes more, until the frosting is smooth, light, and fluffy. (The should be NO lumps!) If the frosting is too stiff to pipe or spread easily, add 1 Tablespoon more cream and beat to combine. If it's too soft, feel free to add more powdered sugar 1 Tablespoon at a time, as needed to thicken it up. 

Frost. Transfer the frosting to a piping bag fitted with an open tip to pipe frosting onto the cookies or use a spatula or cookie scoop to scoop frosting onto the cookies. (Work with just 2-3 cookies at a time, so you can decorate them!) 

Decorate. Pipe the frosting into a spiral shape or spread the frosting with an offset spatula into an even layer. Sprinkle the frosted cookies with more cookie crumbs and top each cookie with 1-2 gummy worms. Repeat until all cookies are decorated. 

Serve & Store. Enjoy right away or store in an airtight container in a single layer at room temperature 2-3 days. These cookies don't freeze well decorated, but can be frozen BEFORE decorating up to 2 months. Thaw frozen cookies and decorate before serving.