Make the cake according to the package directions for a 13x9" cake. Bake and set aside until fully cooled.
Using your desired shape cookie cutter, cut out shapes into the cake and move each piece to a wire rack. Spread 1-2 tbsp. of marshmallow fluff onto one cutout and then top with another piece of cake to make a sandwich.
Meanwhile, add the chocolate chips to a microwave-safe bowl. Microwave for 30 seconds, stir, and then microwave for another 30 seconds. Continue to microwave in 15 second increments until the chocolate is fully melted.
Pour the melted chocolate over top of each whoopie pie. Top with sprinkles and allow the chocolate to completely harden. Store in an airtight container.
by The Savvy Kitchen