In the bowl of a food processor fitted with a dough blade, mix the flour, sugar, and salt. Add in the butter and pulse until the mixture forms pea-size crumbs. Pour in the water a little bit at a time and pulse to mix after each addition. Add just enough water to form a ball of dough. Wrap the dough in plastic wrap and rest it in the fridge for at least 30 minutes and up to overnight.
In a medium bowl, whisk the flour, sugar, cinnamon, and salt together. Mix in the butter, crushing it into the mixture with your hands, until you form crumbs. Reserve the topping for later use.
Preheat the oven to 350 degrees F. Grease a 9-inch pie dish with butter or nonstick cooking spray.
Roll the dough out to ⅛-inch-thick and place in the prepared pie dish, crimping the edges as desired. Rest the pie dough in the refrigerator while you prepare the filling.
Mix together the boiling water, molasses, baking soda, vanilla, salt, and egg. Pour the filling into the pie crust and top with the crumb topping (some of the topping will sink into the pie). Bake until the filling sets and the topping is brown, about 50 minutes.