Roll out your pie crust and fit it to a 9-inch pie pan. Crimp or flute edge as desired, prick bottom with a fork, then place in the freezer to chill for 30 minutes. Meanwhile, preheat the oven to 400°F.
Once the crust is chilled, place a piece of foil onto the crust, pressing to cover the whole crust and edges. Pour in a layer of pie weights or dried beans. Bake for 20 minutes, then carefully remove the foil with the beans, and bake 5 minutes more. Remove from the oven and let cool slightly while you prepare the filling. Reduce the oven to 350°F.
In a bowl, whisk together molasses and water, then whisk in baking soda until foamy. Lightly beat egg and add to mixture along with vanilla, whisking the entire time to prevent the egg from cooking.
In another bowl, combine flour, brown sugar, and butter. Use a pastry cutter, a fork, or your fingers to blend together until the mixture resembles wet sand and clumps when squeezed together.
Pour the molasses mixture into the prepared crust. Then, carefully spoon over the topping in an even layer. Do not do this too quickly, as you want it to float on the top, not sink. If using a pie shield, place it on before baking.
Carefully place the pie into the oven on the center rack. On the lower rack, place a rimmed baking sheet to catch any drips.
Bake for 45-50 minutes, until golden brown and puffed. Let cool completely before serving.
Notes
If using packaged crust, follow package instructions for parbaking.