Add the sugar, butter, cocoa, eggs, vanilla, and salt to a large mixing bowl. Whisk together until well combined. Stir in the flour and chopped pecans.
Pour the batter into a greased and floured 15x10-inch pan.
Bake for 20-25 minutes or until a tester inserted in the center comes out clean. Remove the cake from oven and top evenly with the marshmallows. Return to the oven and bake for 5 more minutes. While the cake is baking, make the frosting.
Place all ingredients in a medium bowl. Beat at medium speed with an electric mixer until combined and smooth.
Remove cake from the oven and drizzle chocolate frosting over warm cake.
Allow cake to cool completely before cutting.
For a shortcut, you can substitute two packages of brownie mix for the cake layer.Allow the cake to cool completely before cutting it. If you let it cool completely, even refrigerate it, before cutting it will look so much nicer.