Make the crust: In a large bowl, whisk together the flour and salt. Cut in the cold butter using a pastry blender or your fingertips until the mixture resembles coarse crumbs. Gradually add the ice water, mixing until just combined. Do not overmix.
Chill the crust: Wrap the dough in plastic wrap and refrigerate for at least 30 minutes.
Prepare the filling: In a medium bowl, whisk together the molasses, hot water, corn syrup, egg, vanilla extract, baking soda, cinnamon, and nutmeg until well combined.
Make the streusel topping: In a separate bowl, combine the flour and brown sugar. Cut in the cold butter using a pastry blender or your fingertips until the mixture resembles coarse crumbs.
Assemble the pie: On a lightly floured surface, roll out the chilled pie crust dough to a 12-inch circle. Transfer the dough to a 9-inch pie plate. Trim and crimp the edges of the crust.
Fill the pie: Pour the molasses filling into the prepared pie crust.
Add streusel topping: Sprinkle the streusel topping evenly over the filling.
Bake: Bake the pie in a preheated oven at 375°F (190°C) for 45-55 minutes, or until the crust is golden brown and the filling is set. The center might still be slightly jiggly, but it will firm up as it cools.
Cool: Let the pie cool completely on a wire rack before slicing and serving.