01 - Preheat oven to 350°F (175°C). Line a 9x9-inch baking pan with parchment paper.
02 - In a medium bowl, mix graham cracker crumbs with melted butter until evenly combined. Press firmly into the bottom of the prepared pan.
03 - In a large bowl, whisk pumpkin puree, eggs, vegetable oil, brown sugar, and milk until smooth.
04 - Add flour, baking powder, baking soda, salt, cinnamon, ginger, and nutmeg to wet mixture. Stir until just combined without overmixing.
05 - Pour pumpkin muffin batter evenly over crust, then sprinkle chocolate chips over the top.
06 - Bake for 25 minutes until almost set.
07 - Remove pan and scatter mini marshmallows evenly over the surface. Return to oven for 5–7 minutes until marshmallows are golden and bars test done with a toothpick (avoid chocolate areas).
08 - Let bars cool completely in the pan before dividing into squares for serving.