• Preheat oven to 350 degrees and spray 9 x 13 pan with non-stick spray.
• Cream butter and sugar in a large bowl.
• Add eggs, one at a time and vanilla and mix until smooth.
• Add flour, cocoa powder, baking powder and salt and mix just until combined. Fold in nuts, if using.
• Pour batter into prepared pan and bake for 20-25 minutes or until toothpick inserted in center comes out mostly clean.
• Pour 1/2 bag marshmellows over the brownies and return to the oven and bake another 3-5 minutes or until marshmellows are melted.  Remove from the oven.
• Let stand until almost cooled.  In the meantime make the frosting.

Frosting:

• Mix 1/2 stick butter, 1/3 cup evaporated milk, 1/3 cup cocoa powder, 3 cups powdered sugar and 1 tsp vanilla together until smooth and creamy with hand mixer.
• Pour over the top of the brownies and spread evenly with an offset spatula or knife.  Let frosting set and enjoy!
• Notes: *Store brownies at room temperature in foil covered container for about 2 days. To keep them fresh, don't cut them until ready to serve. *If you plan to freeze the brownies, bake the brownies and melt the marshmellows on top, then frost when you are ready to serve.  *To freeze brownies pack tightly in a freezer bag or covered freezer safe container.  Freezing brownies can change the texture of them.