Prepare the pan: Preheat the oven to 350°F (175°C). Grease and line an 8x8 baking dish with parchment paper.
Make the crust: In a medium bowl, mix the graham cracker crumbs, melted butter, and sugar until combined and the texture resembles wet sand. Press the mixture firmly and evenly into the bottom of the prepared pan.
Make the filling: Add the pumpkin puree, sweetened condensed milk, eggs, cinnamon, ginger, nutmeg, and salt to a large mixing bowl. Whisk until smooth and fully combined.
Bake: Pour the filling over the crust. Bake for 35 to 40 minutes, or until the edges are set but the center is still slightly jiggly, like cheesecake. Do not overbake.
Cool: Let the bars cool completely at room temperature, then refrigerate for at least 2 hours to fully set.
Make the ganache: Place the chocolate chips in a medium heatproof bowl. In a small saucepan, heat the heavy cream over medium heat until it just begins to simmer (do not boil). Pour the hot cream over the chocolate chips, cover, and let sit for 5 to 10 minutes. Mix together until smooth and glossy.
Pour the ganache over the chilled pumpkin pie layer and spread evenly. Allow to set for 20 to 30 minutes at room temperature or in the fridge until slightly firm.
Add marshmallows: Sprinkle mini marshmallows over the ganache. Use a kitchen torch to toast until golden brown, or place under the broiler for 1 to 2 minutes, watching carefully.
Slice & serve: Allow the ganache to set slightly, then lift the bars out of the pan using the parchment paper. Slice into squares and enjoy chilled or at room temperature.