Preheat the oven to 350° degrees
In a large bowl, whisk together the melted butter and the next 5 ingredients.
Stir in flour and chopped pecans.
Spread the dough (it will be thick) in a greased and floured 15 x 10-inch jelly pan (or use a deep dish cookie sheet).
Bake at 350° for 20 to 25 minutes or until a wooden pick inserted in the center comes out clean. Note: Reduce the temperature by 25 degrees if using a dark baking pan.
Remove from oven; cover hot cake evenly with marshmallows.
Return to oven and bake for 5 minutes.
Prepare the chocolate frosting: Beat all ingredients together with an electric mixer until smooth.
Pour the chocolate icing over the hot cake.
Let the cake cool completely before cutting into squares!