Roll zwieback very fine with a rolling pin—there should be 1 c crumbs.
Add cinnamon and butter, and blend well with a spoon.
Use to line a 9" pie plate, pressing firmly to pie plate.
Beat yolks until light and lemon colored; then gradually stir in condensed milk, lemon rind, and juice.
Fold in stiffly beaten egg whites.
Pour into lined pie plate; bake in a moderately hot oven of 375°F for 25 min.