Preheat over to 350 degree. Mix hot water with jello and set aside. Spray a bundt pan with oil and dust with flour. Shake out excess flour.
In a mixing bowl, combine cake mix, cream cheese, eggs, jello water, sugar, oil and vanilla. Mix on low speed 1 minutes, scraping bottom and sides of bowl as needed. Increase speed to medium and mix for 2 minutes (cake mixes require less beating time).
Pour batter into prepared cake pan and bake 35 - 40 minutes, or when cake top springs to the touch. Cool on rack for 20 minutes. Use a long spreading knife to loosen around edge of cake pan. Invert onto a cake plate. Frost cake with the Chocolate Marshmallow Frosting.
FROSTING: Sift together the sugar and cocoa powder in a mixing bowl. Set aside. Combine the marshmallows, butter, and milk in a medium-size saucepan over low heat. Stir until the marshmallows are melted. Combine the sugar and cocoa mixture with the marshmallow mixture; add vanilla and stir until the frosting is smooth.