In a mixing bowl, combine molasses, hot water, and baking soda. Let cool, then add the beaten egg.
In another bowl, mix flour and brown sugar. Cut in the cold butter until the mixture resembles coarse crumbs.
Pour the molasses mixture into the unbaked pie crust. Sprinkle the crumb mixture evenly over the top.
Bake for 15 minutes, then reduce the oven temperature to 350°F (175°C) and bake for an additional 30 minutes, or until the filling is set and the top is golden brown.
Let the pie cool completely before serving.
Notes:
This pie is best enjoyed the day after it's made, allowing the flavors to meld together.