How to Make It
In a food processor, pulse the flour with the salt. Add the butter and pulse until it is the size of small peas. Add the ice water and pulse until the pastry is moistened. Turn the pastry out onto a floured work surface and knead 2 or 3 times, just until the dough comes together. Pat the pastry into a disk, wrap in plastic and refrigerate until firm, about 30 minutes.
Preheat the oven to 425°. On a lightly floured work surface, roll out the dough to an 11-inch round. Ease the round into a 9-inch pie plate. Trim the overhanging dough to the edge of the pie plate and refrigerate the shell for 20 minutes.
Line the pie shell with parchment paper and fill it with dried beans or pie weights. Bake the shell for about 10 minutes, just until dry. Remove the parchment and beans and bake the crust for about 10 minutes longer, until lightly golden. Reduce the oven temperature to 350° and let the shell cool completely.
In a food processor, combine the flour, maple sugar and salt and pulse to combine. Add 1 tablespoon of the maple syrup and pulse until blended, then add the butter and pulse until the mixture resembles coarse, wet sand.
In a medium bowl, whisk the hot water with the baking soda. Whisk in the remaining cup plus 2 tablespoons of maple syrup and the molasses, then whisk in the egg. Add 1 1/3 cups of the flour mixture and whisk until the filling is uniform in color. Pour the filling into the cooled pie shell and sprinkle the remaining flour mixture (about 2/3 cup) on top.
Bake the pie for 35 to 40 minutes, until golden brown and set. Let the pie cool completely before serving.
The pie can be covered and kept at room temperature overnight.