Ingredients

• cooking spray
• 1.25 cups all-purpose flour
• 1 teaspoon baking powder
• 0.5 teaspoons kosher salt
• 0.5 cups unsalted butter
• 0.5 cups packed dark brown sugar
• 0.67 cups white sugar
• 1 large egg
• 2 large egg whites
• 1.5 teaspoons vanilla extract
• 0.5 cups semisweet chocolate chips
• 0.67 cups mini marshmallows

Instructions

1. 1 Gather all ingredients. Preheat the oven to 350 degrees F (180 degrees C) with a rack in the bottom third position. Spray a 9x9-inch square baking pan with cooking spray, and line with parchment paper, leaving a 2-inch overhang on 2 opposite sides.
2. 2 Whisk flour, baking powder, and salt together in a bowl until well combined. Set aside.
3. 3 Beat butter, brown sugar, and 1/3 cup white sugar in a large bowl with an electric mixer on medium-high speed until light and fluffy, about 3 minutes.
4. 4 Add whole egg and vanilla; beat on medium speed until well combined, about 30 seconds. With mixer running on low speed, gradually add flour mixture, beating just until combined, about 1 minute.
5. 5 Spread mixture into an even layer on the bottom of the prepared baking pan using an offset spatula. Sprinkle semisweet chocolate chips and marshmallows on top.
6. 6 Add egg whites to a large, clean bowl and beat with an electric mixer on medium-high speed until soft peaks form, 1 to 2 minutes. With mixer running on medium-high, add remaining 1/3 cup sugar in a slow, steady stream, beating until stiff, glossy peaks form, 1 to 2 minutes.
7. 7 Spoon the egg white topping over marshmallow layer and spread into an even layer using an offset spatula.
8. 8 Bake in the preheated oven until top is golden brown and sides pull away from the edges, about 30 minutes. Cool on a wire rack until pan is cool to the touch, about 1 hour.
9. 9 Remove from pan using parchment paper overhang; transfer to a cutting board. Cut into squares. Serve.

Nutrition per serving