Preheat the oven to 350F. Line 2 large baking trays with parchment paper or silicone baking mats.
Add the butter, oil, eggs, and cake mix to a large bowl. Use a handheld electric beater to mix well. Add the green gel food coloring and beat it in.
Add the powdered sugar and cornstarch to a shallow bowl and mix to combine.
Use a 1-tablespoon scoop to measure out the dough and scoop it directly into the bowl of powdered sugar/cornstarch. Roll the dough ball around to coat, and then place it on the prepared baking tray. Leave a couple of inches between cookies so they have room to spread out.
Bake until the cookies are puffed and set along the outside, about 8 to 10 minutes. If you’re cooking multiple trays at the same time, flip them once halfway through.
As soon as you remove the cookies from the oven, very carefully (the trays are hot!) place 1 candy heart onto each cookie, gently pressing it in.
Let the cookies cool for 5 minutes on the trays, and then transfer to a wire rack to finish cooling.
Notes
We used a standard-sized (15.25 ounce) box of yellow cake mix to make these cookies. You can substitute with white cake mix if preferred.
You can typically find candy heart sprinkles in the baking aisle of most grocery stores, or you can purchase them online. If you work quickly (but carefully) and place these on the cookies right after they’re done baking, they stick very well. However, if you’re having trouble with it, you can use a little bit of frosting as “glue” to adhere them.
If you can't seem to find heart sprinkles then you can substitute with another heart-shaped candy or easily draw the hearts on yourself with red decorator gel (or pipe them on with red tinted frosting). If all else fails, you can dot the top of the cookies with red M&M's and still get that festive Grinch look.
Use green gel food color to find your perfect shade of Grinch green. Start with 1/4 teaspoon and work your way up from there if needed. I highly suggest using gel food coloring over liquid food coloring as the colors are much more vibrant in baked goods.