Lemon Cornstarch Filling

1/2 cup granulated sugar
2 tablespoons cornstarch
1/4 teaspoon salt
2 egg yolks
3/4 cup water
1/3 cup lemon juice
1 tablespoon butter
1 teaspoon grated lemon rind

Put sugar, cornstarch and salt in double boiler. Mix slightly beaten egg yolks with water and lemon juice. Add to sugar mixture. Cook over boiling water about 15 minutes, stirring constantly. Add butter and lemon rind. Cool, then spread between cake layers.