Preheat oven to 375 degrees and line an 8 inch square pan with parchment paper

In a large mixing bowl whisk together melted butter and cocoa powder until smooth.

Stir in sugar using a wooden spoon. Mixture will be thick

Add eggs and vanilla, and whisk until well combined and smooth

Whisk or sift together flour, baking powder and salt. Add to chocolate mixture and whisk to combine.

Stir in nuts if using

Bake at 375 degrees for 25 minutes. Remove from oven and arrange marshmallow halves to cover the brownie base. Return to oven for 3 minutes, then remove from oven and using a fork, gently squeeze down marshmallows. Set aside to cool completely

Combine frosting ingredients and using an offset spatula, smooth frosting over the cooled marshmallow brownies. Refridgerate to firm up frosting.

Cut into equal size squares. 16 large squares or 24-32 small squares for snack size