Preheat oven to 350 degrees F. Grease a 9×13 pan and set aside.

In the bowl of a stand mixer, beat together butter and granulated sugar until creamy. Beat in 1 whole egg and 2 egg yolks (put aside the whites for later.) Beat in the vanilla extract.

In a separate medium bowl, whisk together the flour, salt, and baking powder. Add the dry ingredients to the wet ingredients. Blend until just combined. Spread mixture into 9×13″ pan. Sprinkle with marshmallows and chocolate chips.

In a clean mixing bowl with whisk attachment on your mixer, beat the 2 remaining egg whites until stiff peaks form, about 2 minutes on high.  (If you do this first, and set it aside in a bowl, you can reuse the same bowl for the cookie base without having to wash it)

Fold in 1 light brown sugar and stir until combined and there are no lumps. (If your brown sugar is a bit clumpy, push it through a flour sifter first to get all the lumps out)

Spread on top of chocolate chips and marshmallows.

Bake 30-35 minutes until top is golden brown.

Allow to cool completely before cutting into squares