This is a great dessert – even better served warm with whipped cream.
Preheat the oven to 325 degrees F. Grease a 9-inch by 13-inch pan.
In a large mixing bowl, combine the egg, sugar, shortening, and molasses. Beat until thoroughly combined. Sift together the flour, soda, salt, and allspice. Add to the molasses mixture and beat until well combined. While still beating, gradually add the boiling water. Beat until well combined. Pour into the baking pan.
Bake for 30 to 35 minutes. Serve warm with whipped cream.