Preheat your oven to 400°F (200°C). Fit the unbaked 9-inch pie crust into a standard pie plate, crimp the edges, and place the pie dish on a baking sheet to make it easier to move and to catch any drips.

In a large mixing bowl, whisk together the flour, brown sugar, granulated sugar, cinnamon, nutmeg, ginger, and salt until well combined and no large clumps of sugar remain.

Add the cold butter cubes to the dry mixture. Using a pastry cutter, two knives, or your fingertips, work the butter into the mixture until it resembles coarse crumbs with some pea-sized bits. Do not overwork; a few larger pieces of butter help keep the topping tender and rustic.

Scoop out 1 cup of the crumb mixture and set it aside in a small bowl. This reserved portion will be the crumb topping. Leave the remaining crumbs in the large bowl for the filling base.

In a medium bowl or large measuring cup, whisk together the dark molasses and boiling water until smooth and fully blended. Let the mixture cool for about a minute so it is warm but not scorching hot.

Whisk the baking soda into the warm molasses mixture; it will foam slightly. Then whisk in the egg and vanilla until the mixture is smooth and well combined.

Pour the molasses mixture into the large bowl with the remaining crumb mixture. Stir gently until you have a smooth, pourable batter with no dry pockets. The batter will be fairly thin, which is correct for a sticky "wet-bottom" filling.

Pour the molasses filling into the prepared pie crust. The crust should look quite full but should not overflow. If it reaches the very top edge, leave behind a tablespoon or two of filling.

Sprinkle the reserved crumb mixture evenly over the surface of the filling. Do not press it down; leave the crumbs light and fluffy so some sink into the filling while others stay on top as it bakes.

Bake the pie at 400°F (200°C) for 10 minutes to help set the crust and start the filling. If the crust edges brown too quickly, shield them with a strip of foil.

Without removing the pie from the oven, reduce the oven temperature to 350°F (175°C) and bake for an additional 30–35 minutes. The center should be mostly set but still slightly wobbly like soft gel, and the crumb topping should be lightly golden.

Remove the pie from the oven and place it on a wire rack. Let it cool completely at room temperature for at least 3 hours so the sticky molasses layer can thicken and the pie can set for clean slices.

Slice the cooled pie with a sharp knife, wiping the blade between cuts if needed. Serve at room temperature or slightly warmed, plain or with whipped cream or vanilla ice cream. It pairs especially well with a hot cup of coffee.