Combine flour, sugar, spices and salt; blend in short- ening until crumbly.
Dissolve soda in 3/4 cup boiling water; combine soda mixture, molasses and egg yolk.
Alternate layers of crumb mixture and egg mixture in pastry shell; ending with crumb mixture on top.
Bake at 450 degrees until crust edges start to brown.
Reduce oven temperature to 350 degrees; bake for 20 minutes longer.