• Preheat oven to 350 degrees F. Line a 13x9 inch metal pan with parchment paper or foil. If using foil, grease the foil.
• In a large heavy saucepan, melt butter and chocolate together. Let cool slightly, then whisk or use a heavy duty scraper to stir in the sugar, eggs, and vanilla. Mix together the flour, baking powder and salt (if using), then add to the batter, stirring until well combined. Spread in the pan.

Cream Cheese Layer

• In another bowl, using a heavy duty scraper, beat together softened cream cheese, softened butter, salt (if using) and sugar until fluffy. Add the egg, vanilla and flour and stir just until mixed. Stir in nuts and spread batter in the pan (just drop it on and smooth gently with spoon) covering the chocolate mixture. Sprinkle with chocolate chips.
• Bake at 350 F for 25 to 30 minutes. Remove from the oven and sprinkle with marshmallows. Return to oven for 2 minutes to puff marshmallows.

Frosting

• While the brownies are baking, have all your frosting ingredients ready to go. Clean out the saucepan used for the batter and melt the chocolate and butter together in that saucepan. Let cool slightly.
• Add the softened cream cheese to the melted chocolate mixture and stir until smooth, then add milk, vanilla and powdered sugar and beat by hand until smooth. Pour warm frosting over the marshmallow layer and swirl.
• Cool completely in the pan, then transfer to the refrigerator and chill until firm enough to slice neatly.
• When ready to cut, lift from pan and set on a big cutting board. With a chef's knife, trim the edges where the chocolate frosting did not reach. Now score the cake in half, then score each half into 12 bars so that you get 24 bars total.