Heat the oven to 350°F. Coat an 8x8-inch square baking pan with cooking spray and line it with parchment paper.

In a stand mixer, beat the unsalted butter, granulated sugar, and kosher salt together on medium speed until light and fluffy, about 2 minutes.

Reduce the speed to low, add the egg and vanilla extract, and beat until combined, scraping the bowl as needed.

Turn off the mixer, add the all-purpose flour and baking powder, and beat on medium speed until just combined.

Spread the dough into the prepared baking pan. Sprinkle chocolate chips and marshmallows evenly over the top.

In a clean mixer bowl, beat the egg whites, brown sugar, vanilla extract, and pinch of kosher salt on high speed until medium, shiny peaks form, about 4 to 5 minutes.

Spread the meringue over the cookie base in an even layer.

Bake until golden-brown and the top begins to crack, for 28 to 30 minutes.

Cool on a wire rack until just warm or at room temperature, at least 1 hour.

Using the excess parchment, lift the slab out of the pan and cut into 9 pieces.