Preheat the oven to 350 degrees F.
Line a 9x9 inch square baking dish with parchment paper. Set aside.
In a large mixing bowl, cream the butter until smooth.
Add the brown sugar and beat until fluffy.
Next, add the egg and vanilla extract and mix until well combined.
Add the flour, baking powder, and salt and mix until well combined.
Fold in the graham cracker crumbs, about ¾ cup chocolate chips, and about ¾ cup of marshmallows.
Spread evenly into the prepared baking dish.
Break the chocolate bars into pieces and add them to the top of the batter, then sprinkle with remaining chocolate chips and marshmallows.
Bake for 20-25 minutes or until a toothpick inserted in the center comes out clean.
Cool completely on a wire rack.
Remove from the pan and cut into squares.
Store in an airtight container.