Some crumb topping sinks as the pie bakes, making a cakelike layer between the molasses custard on the bottom and the sugary topping. Serve with a dollop of lightly sweetened whipped cream.
For the Crust
- 1 1/4 cups all-purpose flour, plus more for rolling
- 1/2 teaspoon coarse salt
- 1 teaspoon granulated sugar
- 1/2 cup (1 stick) cold unsalted butter, cut into small pieces
- 3 to 5 tablespoons ice water
For the Topping and Filling
- 1 1/4 cups all-purpose flour
- 3 tablespoons cold butter, cut into small pieces
- 3/4 cup light-brown sugar
- 1 teaspoon baking soda
- 2/3 cup boiling water
- 3/4 cup unsulfured molasses
- 1 large egg, lightly beaten
Make crust: In a food processor, pulse flour, salt, and sugar until combined. Add butter; pulse until mixture resembles coarse meal, with a few pea-size pieces of butter remaining. Sprinkle with 3 tablespoons ice water. Pulse until dough is crumbly but holds together when squeezed (if necessary, add up to 2 tablespoons water, 1 tablespoon at a time). Do not overmix. Form dough into a disk, wrap tightly in plastic, and refrigerate until firm, at least 1 hour (or up to overnight).
Preheat oven to 375 degrees. On a floured work surface, roll out dough to a 12-inch round with a floured rolling pin. Place in a 9-inch pie plate, trim excess, and crimp edges. Make topping and filling: In clean bowl of processor, pulse flour, butter, and brown sugar until combined. In a bowl, combine baking soda with boiling water and whisk to dissolve. Whisk in molasses, then egg until combined.
Fill crust with molasses mixture, then sprinkle crumb mixture evenly over top (some will sink into filling). Place pie on a rimmed baking sheet and bake until set in middle, 45 to 55 minutes. Let cool 1 hour. Serve warm or at room temperature.