• Preheat oven to 350 degrees.
• Line a 15x10x1 inch sheet pan with foil or parchment paper. Spray with non-stick spray.
• Microwave 1 cup butter and chocolate in a microwave-safe bowl at 30 second intervals until melted. Stir to combine and allow to cool.
• Combine sugar, flour, cocoa and salt.
• Combine eggs and vanilla with cooled chocolate.
• Add egg mixture to flour mixture and mix until combined and smooth.
• Pour onto prepared sheet pan.
• Bake at 350 degrees for 20 minutes.
• Carefully remove from oven and spread marshmallows evenly over brownie layer.
• Return to oven and bake 8 to 10 minutes to toast marshmallows.
• Remove from oven and immediately sprinkle with 3 Musketeer candy bars.

Make Chocolate Frosting:

• Combine butter, cocoa and milk in a small sauce pan over low heat. Cook 2 to 3 minutes until butter is melted and mixture thickens slightly.
• Add confectioners sugar one cup at a time, whisking to combine.
• Add vanilla and whisk until smooth.
• Pour over slightly cooled cake.