In a saucepan, combine the ingredients for the molasses layer. Cook and stir over medium heat until the mixture has thickened. This will take about 5-10 minutes, depending on your stove. Allow it to come to a low boil, but not a full boil. Allow the mixture to cool while you prepare the topping.
For the topping, combine the flour, brown sugar, melted butter and baking soda. Mix well.
Pour the molasses mixture into the unbaked pie shell.
Sprinkle the crumbs on top of the molasses mixture.
Bake at 375 for 40-45 minutes until the top is golden brown and the center starts to puff up.
Allow the pie to cool before slicing. This is considered a "wet-bottom" pie, so when you slice it, the bottom may be slightly "wet" and that's OK.
Notes
*I like using dark brown sugar in this recipe for an extra boost of flavor. *I also think that adding a touch of cinnamon in the crumb topping would be delicious!