Boil 5 cups of water (do 2 1/2 cups at a time). Dissolve 1 package Jello with 1 cup boiling water; add 1/2 cup cold water. Pour into chilled Pyrex dish and return to refrigerator until very firm.
Boil 2 cups milk with 1 cup sugar. Remove from stove. Dissolve 2 packets Knox plain gelatin in 1/2 cup cold water. Add to hot milk-sugar mixture, stir and add 1 pint sour cream and 2 teaspoons vanilla. Beat until smooth. Cool. Pour 1 1/2 cups of this mixture over first layer of Jello and refrigerate again until second layer is hard. Repeat until all Jello and milk mixtures are used, ending with Jello layer. Cut in 2 inch squares and serve.
