In a large mixing bowl combine all of the ingredients together with a whisk or a spatula to a smooth batter~ it will be thick like brownie batter, more like a dough
Refrigerate the dough for 30 minutes & once chilled, Preheat the oven to 350°F & then scoop balls of dough approximately 1 Tbs or use a #100 scoop then line them up on a parchment lined sheet pan spaced 2 inches apart
Bake for 10-15 minutes *I think mine took 12 minutes to very light golden~ do not over bake or the cookies will be dry. I like mine slightly chewy & soft inside.
Next prepare your favorite recipe for vegan buttercream and then choose your flavors and sprinkles.
Once the cake mix cookies are fully cooled fill the with buttercream and then sandwich them together, roll in sprinkles, nuts, toasted coconut etc....
Notes
Birthday Cake Whoopie Pies will stay fresh for up to 5 days in an airtight container at room temperature. You can freeze them for up to 2 months wrapped well
Recipe by Gretchen's Vegan Bakery at https://www.gretchensveganbakery.com/whoopie-pies/