Preheat the Oven: Adjust a baking rack to the middle of the oven and preheat to 375°F (190°C / gas mark 5).
Prepare Baking Pan: Grease a 9 x 13-inch (23 x 33 cm) baking pan with butter. Line with parchment paper, allowing the ends to extend over the sides for easy bar removal.
Make the Crust: In a large mixing bowl, whisk together the all-purpose flour, brown sugar, baking powder, ground cinnamon, and chopped pecans. Add the softened butter and mix until a crumbly dough forms.
Form the Bottom Layer: Reserve 1 1/2 cups of the dough mixture for the topping. Press the remaining dough firmly into the prepared baking pan to form the crust.
Prepare the Pumpkin Filling: In a separate mixing bowl, combine the canned pumpkin, sweetened condensed milk, eggs, cornstarch, cinnamon, ginger, and nutmeg. Whisk until smooth and well blended.
Assemble the Bars: Pour the pumpkin filling over the crust and spread it evenly. Sprinkle the reserved crumb topping over the pumpkin layer.
Bake: Bake for 28 to 34 minutes, or until the top is golden brown.
Cool and Chill: Remove the pan from the oven and place on a wire rack to cool to room temperature. Then chill in the refrigerator for at least 3 hours or overnight.
Slice and Serve: Use the parchment paper to lift the bars out of the pan. Cut into 18-24 squares with a sharp knife, and enjoy!