Heat the oven to 400°F and set the unbaked crust on a rimmed sheet pan.
Rub the flour, brown sugar and cold butter together until coarse crumbs form.
Whisk the molasses, hot water, baking soda, egg and salt in a separate bowl until foamy.
Pour the molasses mixture into the crust and scatter the crumbs evenly over the top without stirring.
Bake for 15 minutes, lower the oven to 350°F, and bake 25 to 30 minutes more until the crumb top is set but the center still gives slightly.
Cool for at least 3 hours before slicing.