For the pie: Preheat the oven to 350 degrees F. Unroll the pie dough and fit into a 9-inch glass pie plate. Trim the edges if necessary and crimp. Place on a baking sheet.
Combine the molasses, flour and egg in a bowl. Dissolve the baking soda in the hot water and stir into the molasses. Pour the filling into the pie crust.
For the topping: Mix together the flour, brown sugar and granulated sugar. Rub the butter into the flour until crumbly; the mixture will be like sand. Sprinkle over the pie filling. Bake until the filling is set and the edges of the crust are browned, about 50 minutes. Let cool completely or up to overnight before cutting. Serve with whipped cream, if desired. Store loosely covered at room temperature.