Preheat the oven to 350 °F (177 °C) Add parchment paper or silicone baking mats to two cookie sheets.
Beat the eggs, sugar, light brown sugar, butter, corn syrup, and vanilla until fluffy. In a different bowl, whisk the cocoa powder, flour, salt, cornstarch, and baking soda together. Slowly beat the dry ingredients into the wet ones to form the cookie dough.
Use a cookie scoop and scoop the dough into your hand and then roll it into a ball. Place on the cooking sheet and then press down to make a thick flat cookie.
Bake the cookies in the oven for 15-18 minutes. Let them cool completely.
Use the mixer to beat the powdered sugar, butter, whipping cream, and vanilla until a peak forms. Scoop it into a frosting bag and spiral it on top of the cookie. Sprinkle chopped-up heath bar candies on top.