Preheat oven to 400°F. Beat the egg yolk in a small bowl. Blend in the molasses. Add the boiling water once you have dissolved the soda in it. Set aside.
Combine dry ingredients. Add butter and work into crumbs with your fingers. Pour liquid into pastry crust and evenly sprinkle crumbs on top.
Bake for 10 minutes at 400°F, then lower heat to 325°F and bake for another 30 minutes. NOTE: If I had an 8-inch pie plate, I think it would be a better fit for the recipe, but this is how my mother wrote it out.