Pre-heat oven to 350 deg.F.Line a baking pan with parchment paper.Set aside.In a small bowl, whisk the flour,cocoa powder, baking powder and salt together. Set aside.In a medium saucepan, melt the butter until smooth. Remove from heat and stir in the sugar, eggs and vanilla extract.Whisk smoothly.Add the dry ingredients to the butter mix thoroughly. Fold in chopped walnuts.Pour batter into the prepared pan and smooth the top. Bake for 15 minutes and bring the pan out of the oven for the topping.

Sprinkle marshmallows, chocolate chips and nuts evenly on top of the brownie. Return pan to oven and bake for 5 more minutes. If desired, carefully broil for just a minute to brown the marshmallows perfectly.Remove from oven and cool in pan for 5 minutes. Serve brownie warm and store completely cooled leftovers in a cake tin.