Preheat the oven to 350 degrees. Prep a cookie sheet with a sheet of parchment paper. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment, mix together the eggs, oil, vanilla, cake mix, and green gel coloring. Don't overmix! Place in the fridge for 20-30 minutes to firm.
In a small bowl, whisk together the powdered sugar and cornstarch.
With a small cookie scoop, scoop the green cookie dough into a ball, roll it in your hands, then roll it in the powdered sugar + cornstarch mixture. Place onto a cookie sheet. Repeat with remaining dough (I fit about 12-15 on one sheet, but make sure to space them out about 4 inches).
Use the bottom of a flat measuring spoon or cup and flatten the top of the cookies VERY slightly (just a little). Then place on a single heart sprinkle on top.
Bake for 7-9 minutes until they are still a little gooey, but not turning brown yet (I had a batch sit in for about 1 minute too long and got a little toasty!). Let cool on the cookie sheet for 30 seconds, then transfer to a wire rack to cool completely. Store in an airtight container for up to a week.