In a medium bowl, with an electric beater on medium speed, cream the butter and sugar. Add 1 whole egg and 2 egg yolks, reserving the egg whites for later use. Beat until well mixed. Add the flour, baking powder, and salt, mix until thoroughly combined.
Spread the mixture into the bakingdish and sprinkle with the nuts, chocolate chips, and marshmallows.
In another medium bowl, with an electric beater on medium speed, beat the reserved egg whites until stiff peaks form, then fold in the brown sugar. Spread over the top of the batter and bake for 30 to 35 minutes, or until golden.
Remove from the oven and allow to cool, then cut into bars and serve.