Preheat the oven to 350 degrees F. Grease and line a 9x13-inch baking pan with parchment paper. Allow some of the parchment paper to extend above the pan a bit along the long sides. This will help when you go to remove the bars from the pan. Set aside.

In a large bowl, whisk together the flour, graham cracker crumbs, baking powder and kosher salt. Set aside.

In the bowl of a stand mixer, beat together the butter and brown sugar until light and fluffy. Scrape down sides of bowl as needed. Add eggs and beat until well combined. Reduce the mixer's speed to low and, bit by bit, add in the flour mixture until combined.

Set aside about ¼ of the dough. Press the remaining dough into the bottom of the prepared pan in a thin, even layer. Top with pieces of Hershey's bars, then the marshmallows.

Scatter the reserved dough over the top of the marshmallows and chocolate. It won't cover the entire top. This is exactly what you want, so that some of the chocolate and marshmallows show through.

Bake in preheated oven until golden brown, about 30-35 minutes.

Set the pan on a wire rack to cool completely, then lift the bars out of pan by grabbing the parchment paper. Remove to a cutting board and cut into squares.