For filling, combine 1/2 cup sugar, cornstarch, and salt in a saucepan over low heat; stir in milk. Cook until thick, stirring constantly. Whisk together the egg and lemon juice. Add a little thickened milk mixture to the egg mixture then return egg mixture to the pan, stirring to blend well. Continue cooking and stirring over low heat for 2 minutes. Add lemon peel, 1 tablespoon butter, and a few drops of vanilla.
For crust, combine shredded coconut with cracker crumbs, 1/2 cup sugar, flour, and 1/2 cup butter. Put half of the coconut mixture in bottom of an 8-inch square baking pan. Pour in filling; top with remaining coconut mixture. Bake at 400° for 25 minutes, or until top is browned. Chill then cut into squares.
Calories per serving, based on 12 servings: 235