Line a 9×13 baking dish with parchment paper that has been lightly sprayed with nonstick baking spray or has been buttered.
Cream together the softened butter, peanut butter, and sugars with an electric mixer until well combined and fluffy. Add eggs one at a time, beating well after each egg, followed by the vanilla extract. Stir in the flour, baking powder and salt until well combined. Press half of the blondie batter into the pan.
Arrange the following on top of the blondie batter in layers. Graham crackers, topped with the chocolate bars, a single layer of the miniature marshmallows, and finally topped with the remaining blondie batter. Bake for 30 minutes and then remove from the oven to cool.
Notes
Make Ahead and Storage Instructions
Store cooled S'mores Bars in an airtight container for 3 to 5 days.