Cooked lemon filling for layer cakes.

Steps

1. Combine flour, sugar and salt in a saucepan and stir in water.
2. Cook over medium heat stirring until thickened. Cover and cook 2 min longer, stirring occasionally.
3. Stir a small amount of hot mixture into egg, then add to remaining hot mixture. Cook 1 min longer.
4. Remove from heat and blend in butter, rind and juice. Cool. Enough to fill an 8-inch or 9-inch layer cake.