Line a baking sheet with parchment paper and set it aside.
Combine the cake mix, eggs, vegetable oil, and vanilla in a large bowl. Add the green food coloring and mix well to incorporate it evenly.
Chill the dough in the refrigerator for 20 minutes.
Preheat the oven to 350°F. Mix the confectioners sugar and cornstarch in a small bowl.
Remove the dough from the refrigerator. Scoop dough into 1 inch balls, roll in the confectioners sugar mixture until completely coated, and place on baking sheet 2 inches apart.
Lightly press down with the bottom of a glass to make the balls have a flatter surface. (Do not press too hard though).
Add the candy heart to the top of the dough ball, gently pressing it down into the dough a litte bit.
Place the baking sheet in the oven and bake for 8-10 minutes. Do not overbake.
Cool on cookie sheet for 3-4 minutes before transferring to a cooling rack to cool completely.
Notes
Store cookies in an airtight container at room temperature.