Preheat your oven to 350°F (175°C). In a mixing bowl, combine graham cracker crumbs, light brown sugar, and melted unsalted butter until well blended. Press the mixture firmly into the bottom of a lined 9x9-inch pan. Bake for about 10 minutes, or until lightly golden. Allow it to cool completely before adding the next layer.

While the crust is cooling, melt semisweet chocolate with heavy cream in a heatproof bowl over simmering water, stirring until smooth and glossy. Spread the chocolate mixture evenly over the cooled crust. Place the pan in the freezer for about 10 minutes to let it set.

In a large mixing bowl, whisk together light brown sugar, ground spices, pumpkin puree, evaporated milk, and vanilla until smooth. Gradually fold in the beaten large eggs until combined. Pour this mixture over the chilled chocolate layer, ensuring even distribution.

Place the pan back in the oven and bake for 45-50 minutes, or until the filling is set and lightly firm to the touch. The edges should puff slightly, while the center should have a slight jiggle. Once baked, remove from the oven and let it cool completely at room temperature.

After cooling, cover the pan with plastic wrap and refrigerate for at least 3 hours or overnight.

When ready to serve, cut into squares and melt additional chocolate. Drizzle over the bars, followed by a generous layer of miniature marshmallows. Use the broiler on low for about 30 seconds to toast the marshmallows for a golden-brown top.