Preheat your oven to 350°F (175°C) and line an 8x8-inch baking pan with parchment paper. In a mixing bowl, combine crushed gingersnap cookies, melted butter, brown sugar, and optional cinnamon. Press the mixture firmly into the bottom of the lined pan, creating an even layer. Bake for 8 minutes, until slightly golden, then remove from the oven and let it cool while you prepare the pumpkin filling.

In a large bowl, whisk together the pumpkin purée, brown sugar, granulated sugar, melted butter, eggs, and vanilla extract until smooth and well combined. In a separate bowl, sift together the all-purpose flour, baking powder, baking soda, ground spices, and salt. Gradually mix the dry ingredients into the wet mixture until just combined, creating a smooth batter for your pumpkin bars.

Carefully pour the pumpkin batter over your pre-baked gingersnap crust in the pan, spreading it evenly to cover the crust completely. Place the pan in the preheated oven and bake for 28-32 minutes. The edges should be lightly golden and the center set; a toothpick inserted into the center should come out clean. Once done, allow the pumpkin bars to cool completely in the pan on a wire rack.

In a saucepan over medium heat, melt the unsalted butter, then add the brown sugar, stirring continuously until the mixture begins to bubble. Carefully add the milk, bringing it back to a gentle boil while stirring. Remove the saucepan from heat, let it cool slightly, then whisk in the powdered sugar and vanilla extract until you have a smooth frosting ready to pour over the cooled pumpkin bars.

To create a delightful gingersnap crumble topping, mix the crushed gingersnap cookies with brown sugar and melted butter in a small bowl until well combined. As soon as the frosting is spread over the cooled pumpkin bars, sprinkle the gingersnap mixture generously on top of the frosting before it sets, adding a crunchy texture and extra flavor.

Cover the pan and chill the pumpkin bars in the refrigerator for about 1 hour. Once chilled, use a sharp knife to slice the bars into squares, enjoying the delightful layered textures. Serve them with a warm beverage for a cozy treat, and savor the heartwarming flavors of these Pumpkin Bars with Brown Sugar Frosting & Gingersnap Crust!