Prep the crust and oven: Heat the oven to 400°F (205°C). Place the unbaked pie crust in a 9-inch pie plate. Crimp edges and chill while you mix the filling and crumbs.
Make the crumbs: In a medium bowl, whisk flour, granulated sugar, brown sugar, cinnamon, ginger, and salt.
Cut in the cold butter with a pastry cutter or your fingers until it forms coarse crumbs. You want a mix of pea-sized bits and sandy pieces.
Mix the wet ingredients: In a separate bowl, whisk the hot water and molasses until smooth. Whisk in the egg, baking soda, and vanilla.
The mixture will foam slightly—that’s good.
Assemble the pie: Sprinkle about one-third of the crumb mixture evenly over the bottom of the chilled crust. Pour the molasses filling over the crumbs. Sprinkle the remaining crumbs evenly on top.
Don’t pack them down; keep them fluffy.
Bake high, then low: Bake at 400°F for 10 minutes to set the crust edge. Reduce heat to 350°F (175°C) and bake 25–35 minutes more. The pie is done when the center jiggles slightly but isn’t liquid, and the crumbs are lightly golden.
Cool completely: Place the pie on a rack and let it cool at least 2 hours.
The center finishes setting as it cools. Slice with a sharp knife for clean wedges.
Serve: Enjoy at room temperature or lightly warmed. It pairs beautifully with coffee, whipped cream, or a scoop of vanilla ice cream.