Whisk together the sugar, cocoa powder, salt, and flour in a medium bowl. In a 2-cup measuring cup, whisk together the melted and cooled butter, eggs, and the vanilla extract until well combined. Pour the egg mixture into the sugar and flour mixture and stir until well combined. Stir in 1 cup of the toasted pecans. Pour the mixture into the prepared 9x13 baking dish and bake until a toothpick inserted in the center comes out clean, about 30 to 40 minutes.
Remove from the oven and top with the mini marshmallows.
During the last 10 minutes baking time for the cake, prepare the chocolate icing by melting the butter and cocoa in a large saucepan over medium heat. Add confectioners sugar and milk and bring to a full boil. Remove the pan from the heat and stir in the vanilla until well combined.
Pour the chocolate icing over the warm, marshmallow topped cake, making sure to cover to the edges of the pan. Top with the remaining pecans.
Let cool, cut into pieces and serve.
Make-Ahead: Store Mississippi Mud Cake in an airtight container or under a cake dome for up to 3 days.