Preheat oven to 350 and spray a 13- by 9-inch baking pan lightly misting.
Place cake mix,melted butter,buttermilk,eggs,and vanilla in large bowl. Blend with an electric mixer for 1 minute scrape down the sides and mix for 2 more. The batter should look well blended and fluffy, pour into your pan. Smooth it out with a rubber spatula, place pan in oven.
Bake the cake until it springs back when lightly pressed with your finger, about 40 minutes. Remove the pan and place it on a wire cooling rack. Spread the marshmallow creme on warm cake. Don't get discouraged work with it...it melts and will spread give it time to adjust.
Prepare the pan frosting now while the cake cools. Starting with a medium saucepan add you butter and melt over low heat, 2 to 3 minutes. Stir in your cocoa powder and milk. Let the mixture come just to a boil (don't overdue) stirring and then remove pan from the heat and stir in your confectioners sugar until the frosting is smooth and has thickened.
Pour the hot frosting over the marshmallow creme and spread it out so that the frosting reaches the edges of the cake. Scatter the top with toasted pecans. Cool the cake for 20 minutes before serving.
This cake is like lava when you add that hot frosting to the marshmallow you do get a bubbling effect. The warm chocolate seeps down through the cake and gives it a layer on the bottom adding to an already moist cake. ENJOY!