Preheat your oven to 400°F (200°C) and place a rack in the lower third of the oven. Fit the single 9-inch pie crust into a standard pie dish, crimp the edges, and refrigerate while you prepare the filling and crumb topping.

In a medium bowl, whisk together the all-purpose flour, packed light brown sugar, ground cinnamon, and 1/4 teaspoon kosher salt, breaking up any large sugar clumps. Add the cold cubed unsalted butter and cut it into the dry ingredients using a pastry cutter, two forks, or your fingers until the mixture resembles coarse crumbs with some pea-sized pieces. The crumbs should clump when squeezed, then break apart when rubbed.

In a large bowl, whisk together the unsulphured molasses and hot water until smooth. Let sit for 1–2 minutes so the mixture is warm but not steaming. Whisk in the baking soda and 1/4 teaspoon kosher salt; the mixture may foam slightly. Add the egg and vanilla extract and whisk until fully combined and smooth.

Pour the molasses filling into the chilled pie crust. Sprinkle about half of the crumb mixture evenly over the top of the filling; some crumbs will sink into the filling and create the characteristic "wet bottom" layer. Gently sprinkle the remaining crumbs evenly over the surface without pressing them down.

Place the pie dish on a baking sheet to catch any drips and transfer it to the lower third of the preheated oven. Bake at 400°F (200°C) for 10 minutes, then reduce the oven temperature to 350°F (175°C) without opening the door and continue baking for 30–35 minutes. The center should be mostly set with a slight jiggle, and the crumbs should be golden brown. If the crust edges brown too quickly, tent them loosely with foil.

Remove the pie from the oven and cool on a wire rack for at least 2 hours to allow the filling to set. Serve warm or at room temperature, plain or with whipped cream or vanilla ice cream, and a cup of coffee if desired.