How to Make It
In a food processor, combine the flour with the sugar and salt. Add the pieces of cold butter and pulse just until they resemble small peas. Add the ice water and pulse just until the dough is lightly moistened. Turn the dough out onto a lightly floured work surface and knead it several times, until it comes together. Flatten the dough into a disk, wrap it in plastic and refrigerate until chilled, about 30 minutes.
Preheat the oven to 350°. Butter a 9-by-13-inch glass baking dish. On a lightly floured work surface, roll out the pastry to an 11-by-15-inch rectangle, about 1/8-inch thick. Trim the pastry to a 10-by-14-inch rectangle. Roll the pastry around the rolling pin and carefully unroll it in the prepared baking dish, easing it into the corners. Line the pastry with aluminum foil and fill with pie weights or dried beans. Bake the pastry for about 20 minutes, until it is lightly browned and just set. Remove the foil and weights and bake for 8 to 10 minutes longer, until the pastry is cooked through but not browned.
In a medium bowl, whisk the eggs with the light brown sugar, cinnamon and pumpkin-pie spice. Add the pumpkin puree and whisk until smooth. Whisk in the evaporated milk until the filling is blended. Pour the filling into the prebaked crust and smooth the surface. Bake for about 20 minutes, until the filling is set around the edges but very jiggly in the center.
In a medium bowl, toss the walnuts with the flour and brown sugar. Add the butter and stir until combined. Drop tablespoons of the topping onto the partially baked pumpkin layer. Bake for about 20 minutes longer, until the topping is sizzling and golden and the pumpkin filling is just barely jiggly in the center. Let cool completely, then cut into 16 bars.
Pumpkin-pie spice is a mix of cinnamon, ginger, nutmeg, allspice, mace and cloves.