Preheat the oven to 350F.

In a large bowl, beat the cake mix with the water, oil and eggs (according to package directions) and pour into a well buttered Bundt pan. Bake for 40-45 minutes, or until golden and springy to the touch.

Let cool for about 15 minutes before turning out onto a wire rack to cool completely. When you're ready to assemble it, cut the top off - the part that has domed up over the top of the pan as it baked - with a serrated knife and set it aside. Using a sharp paring knife, cut a trough an inch or so deep, leaving at least half an inch on both sides. Once you've cut two circles, pull the cake out from in between. (Fingers work great for this.)

In a large bowl, beat the frosting for 2-3 minutes, until light and fluffy. Fold in the marshmallow cream. If you like, spoon it into a ziplock bag, snip off one corner, and pipe the filling into the cake trough. (Otherwise, just use a spoon - it works just fine!) Return the top of the cake and carefully invert it onto a plate or serving dish.

If you like, decorate it or warm any leftover filling to drizzle on top and cover with sprinkles. Otherwise leave it completely naked - like a Twinkie.

Serves 8-12.