Prepare cake batter by stirring together cake mix, eggs, water and cooking oil until well blended and only slightly lumpy. Pour batter into a bundt pan coated with BAKING spray (contains flour) or grease and sprinkle lightly with flour. Bake at 350˚for 25-35 minutes. Cook time varies by pan so when the top of the cake starts to firm up, insert a toothpick into the middle of the cake, AS SOON AS it comes out clean, remove the cake. Be sure not to overcook or cake will become dry. It will be nice and moist if you cook it just until it sets!
Once the bundt cake is done, remove from oven and cool. To speed up cooling, you can put the cake in the freezer for 30 minutes. (Note: Make sure you let the cake cool completely or the marshmallow filling will melt and be absorbed into the cake after you add it.)
Once the cake is cooled, leave it in the pan and poke the cake almost all the way to the bottom of the pan (which will be the top of the cake). Move your finger from side to side to widen the cavity and allow in more frosting. Keep poking making your holes about 1 and a half inches apart.
Next, fill up a sandwich bag with marshmallow frosting and cut one of the corner tips of the bag to create a pea-sized hole. Seal the bag and squeeze the bag to dispense the marshmallow frosting into each hole. Tip: Make sure to put the tip of the fluff bag as far into each hole as you can get so plenty of fluff gets into each poked hole.
Using a tiny rubber spatula or butter knife, separate the sides of the cake from the bundt pan as best you can without cutting into the cake. Once the cake is nice and loose, place a flat plate or cake stand upside down on top of the bundt cake pan. Then gently turn both the bundt pan and cake pan over and let gravity do its thing. Carefully lift the bundt pan while you peak around all sides to see if the cake dropped evenly onto the plate.
Top with sprinkled powdered sugar for a festive look.