1Preheat oven to 350 degrees F (175 degrees C). Butter and flour two 8-inch round cake pans.
2Beat 1/2 cup melted and cooled butter and eggs together in a bowl with an electric mixer. Add yellow cake mix, water, and pudding mix; beat on medium speed until batter is smooth and thick, about 1 minute. Divide batter between prepared cake pans.
3Bake cakes in the preheated oven until a toothpick inserted in the center comes out clean, 20 to 25 minutes. Cool cakes for 5 minutes in the pans before transferring to wire racks to cool completely.
4Beat 1/2 cup softened butter and vanilla extract together in a bowl with an electric mixer. Add marshmallow creme and beat until smooth. Gradually beat in confectioners' sugar until just combined. Pour whipping cream into marshmallow creme mixture, increase speed to high, and beat until frosting is smooth and fluffy, about 1 minute.
5Place one cooled cake round on a plate. Spread about 1/3 of the frosting over the top of the cake. Place second cake on top of frosting layer. Spread frosting on top and sides of cakes.