In a bowl, dissolve the baking powder in the boiling water. Stir in molasses. Add the beaten egg and whisk until well blended.
In a separate bowl, combine the flour, brown sugar, and a dash of salt. Cut in the cold butter until the mixture resembles coarse crumbs.
Pour the molasses filling into the crust and sprinkle the crumb mixture over all.
Place the pie on a baking sheet and carefully place it in the oven. Bake at 425° for 15 minutes. Reduce the oven temperature to 350° F and continue baking for 25 to 35 minutes, or until the filling is set.