Preheat oven to 350 degrees. Spray the inside of a Bundt pan or cake pan with cooking spray.
Prepare cake batter per instructions on back of box. (I substitute milk for water and melted butter for the oil to make a richer bakery style cake.)
Pour batter into baking dish and spread evenly.
Bake in oven per instructions on back of box.
Remove cake from oven and allow to cool completely in the pan.
Using a utensil with a large rounded end, poke holes throughout cooled cake. (Do not poke holes all the way through bottom of cake.)
To make the filling, mix together the marshmallow fluff and vanilla frosting until smooth.
Using a piping bag with a large tip or a ziploc bag (with a large hole cut in the corner pipe the filling into the holes from the bottom up.
Using a knife or spatula separate cake from pan edges. (if using round cake pans or bundt pan.) Place a plate over the top of baking pan and invert the cake onto the plate. Sprinkle with powdered sugar, if desired. Cut into slices and enjoy.
