Roll out the pie dough and turn into a 9-inch pie plate. Trim and flute the edges.
In a mixing bowl, combine the flour, brown sugar, and softened butter. Mash with a fork until it reaches a consistent, crumbly consistency.
In a separate bowl, beat together the molasses, egg and baking soda with a large spoon until blended. Stir in the boiling water and mix thoroughly (this will begin to foam).
Stir half the crumb mixture into the molasses mixture and pour into the crust. Sprinkle the remaining crumb mixture evenly over the top.
Bake at 400 degrees Fahrenheit, on the center rack, for 10 minutes. Reduce the heat to 350 degrees and bake until the pie filling has puffed around the sides and is firm in the center, about 20 to 30 minutes more. Cool on a rack.