Preheat the oven to 350 degrees F. Spray 2 (10-inch) baking pans with nonstick baking spray with flour.
In a large bowl, beat the sugar and 1 cup melted butter with an electric mixer on medium speed. In another large bowl, add the eggs and 1 teaspoon vanilla and beat on medium-high speed until the mixture is pale and fluffy. Slowly add butter mixture, beating until combined.
In a medium bowl, combine the flour, baking soda, salt, and cocoa. Gradually add to the sugar mixture, alternately with the buttermilk, beginning and ending with the flour mixture. Beat until just combined after each addition. Slowly add the boiling water, beating until just combined. Spoon the batter evenly into the prepared pans, and bake until wooden pick inserted in center comes out clean, about 28 to 34 minutes. Let cool in the pans for 10 minutes. Remove the cakes and let them cool completely on wire racks.
Spread Mississippi Mud filling evenly between layers. Chill the cake for at least 4 hours. Spread Chocolate Cream Cheese Frosting evenly over top and sides of cake. Just before serving, sprinkle marshmallows and 3/4 cup pecans evenly over the top and drizzle with fudge sauce. Store the cake, covered, in refrigerator.
Mississippi Mud Filling:
In a medium bowl, combine the whipped topping, marshmallow creme, and 4-oz cream cheese. Beat at medium speed with an electric mixer until smooth. Stir in 1 cup pecans, then cover, and chill for at least 1 hour.
Chocolate Cream Cheese Frosting:
In a large bowl, beat 1 cup softened butter and package cream cheese at medium-high speed with an electric mixer until creamy. Add the melted chocolates, beating until combined. Gradually add confectioners' sugar, beating until smooth. Add a teaspoon vanilla, beating until incorporated. Use the frosting immediately.