Prepare the oven and pan: Heat the oven to 350°F and butter a 9-by-13-inch baking pan.
Make the batter: In a mixing bowl with an electric mixer, beat the butter and sugar for 3 minutes or until light and fluffy. Beat in the whole egg, 2 egg yolks, and vanilla (you’ll be using the whites later).
Combine the flour, baking powder, and salt in another bowl and stir to blend. Add the dry ingredients to the first mixture and mix on low until combined.
Make the brown sugar meringue: In a separate bowl, beat the egg whites until stiff; fold in the brown sugar.
Assemble the layers in the pan: Spread the batter in the baking pan and sprinkle with the nuts, chocolate chips, and marshmallows. Spoon the meringue evenly over the marshmallow layer and gently spread to cover. Bake for 25 to 35 minutes or until golden brown.
Cool and serve: Cool the bars in the pan on a rack, then cut and serve. Enjoy!