Preheat the oven to 425°F (220°C). Place your prepared pie crust into a 9-inch pie plate and set aside in the fridge until ready for it.
Make the crumb topping: In a medium bowl, whisk together the flour, brown sugar, salt, and cinnamon. Add the cold vegan butter and rub it in with your fingers until you get chunky, uneven crumbs with some large pea-sized pieces. Set aside in the fridge until you're ready for it so it's nice, firm and cold and stays on top of the pie rather than sinking in.
Prepare the molasses filling: In a heatproof measuring jug or bowl, add the molasses. Stir in the baking soda directly into the molasses until fully combined (the mixture will slightly lighten in color).
Carefully pour in the boiling water while stirring continuously. The mixture will foam and bubble—this is normal.
Stir in the vanilla extract and mix until smooth and fully combined.
Carefully pour the molasses mixture into the cold unbaked pie crust. Sprinkle the crumb topping evenly over the surface without pressing it down.
Add your pie dish to your oven and a baking sheet in the pan underneath, or add your pie dish on top of a baking sheet (either one works) to prevent any drippings burning onto the bottom of your oven.
Bake at 425°F (220°C) for 10 minutes to quickly set the top. Without removing the pie, reduce the oven temperature to 350°F (175°C) and continue baking for 35–40 minutes, until the crumb topping is deep golden and the filling looks set with only a very slight jiggle in the center.
If the crust edges brown too fast, loosely cover them with foil.
Remove from the oven and cool on a rack for at least 1–2 hours. The filling firms up as it cools—cutting too early will give you a loose center.
Notes
Room temperature: Cover shoofly pie loosely and store at room temperature for up to 2 days.
Refrigerator: Store in the fridge for up to 5 days.