Make the crust: Pulse the flour, brown sugar and salt in a food processor to combine. Add half of the butter and pulse until combined. Add the remaining butter and pulse until the mixture looks like coarse meal. Add 1/4 cup ice water; pulse a few more times until the dough just holds together when pinched (add up to 1 more tablespoon ice water if needed). Turn the dough out onto a piece of plastic wrap; press into a disk and wrap. Refrigerate until firm, at least 1 hour or overnight.
Meanwhile, make the filling: Preheat the oven to 350 degrees F. Spread the pecans on a baking sheet and bake until lightly toasted, about 8 minutes. Let cool completely; set aside. Set another baking sheet on the middle oven rack.
Put the dates in a small bowl and cover with 1/2 cup boiling water; set aside 5 minutes. Transfer the dates with their soaking liquid to a food processor. Add the butter and pulse until smooth. Add the pumpkin puree, corn syrup, brown sugar, vanilla, flour and cinnamon; pulse until smooth. Add the heavy cream and eggs; pulse until just combined.
Roll out the dough into a 12-inch round on a lightly floured surface. Ease into a 9-inch pie plate, pressing the dough into the bottom and up the side. Fold the overhanging dough under itself and crimp with your fingers or a fork. Refrigerate 15 minutes.
Sprinkle 3/4 cup of the toasted pecans in the prepared crust and add the filling. Transfer to the hot baking sheet in the oven and bake until the filling is set around the edge but the center still jiggles, about 1 hour. Transfer to a rack and let cool.
Make the toffee sauce: Bring the butter, brown sugar, corn syrup, heavy cream and salt to a boil in a small saucepan. Reduce the heat to medium low and simmer until dark and thick, 8 to 12 minutes. Remove from the heat and let cool 3 minutes. Pour the warm toffee sauce over the pie and sprinkle the remaining 1/2 cup toasted pecans on top. Let sit until the toffee sauce sets, at least 1 hour.