Combine melted butter and cocoa powder. Stir until smooth.
Cream brown sugar and white sugar with the butter/cocoa mixture.
Add eggs and vanilla.
Stir in salt, baking soda, and flour.
Mix in chocolate chips and 1 cup chopped pecans.
Chill dough for 30 minutes. Preheat oven to 350 degrees.
Scoop dough onto cookie sheets using a medium cookie scoop.
Bake cookies for 10 minutes. Top each cookie with 6 mini marshmallows then cook an additional 2 minutes. DO NOT cook longer than 2 minutes!
Mix frosting ingredients and beat until smooth and creamy. Add additional milk if necessary.
Allow cookies to cool then spread with chocolate frosting and sprinkle with additional nuts.