Preheat oven to 350℉. Line a 9x13 baking pan with parchment paper; set aside.
In a large bowl, cream the butter, granulated sugar, 1 whole egg plus 2 egg yolks (SAVE THE WHITES!), and vanilla until smooth.
½ cup (113g) butter, softened, 1 cup (210g) granulated sugar, 1 teaspoon vanilla
Add flour, baking powder, and salt. Mix just until combined.
1½ cups (215g) all-purpose flour, 1 teaspoon baking powder, ½ teaspoon salt
Press in an even layer into the prepared baking pan.
Sprinkle with marshmallows and chocolate chips.
2 cups (110g) mini marshmallows, 1 cup (180g) semi-sweet chocolate chips
In a clean mixing bowl with whisk attachment, beat the 2 egg whites until stiff peaks form. Fold in the brown sugar until combined and there are no lumps remaining.
1 cup (195g) packed brown sugar
Spread evenly over marshmallows and chocolate chips. Bake for 25-30 minutes or until top is light brown. Cool completely before cutting into bars.