Preheat oven to 350º. In a saucepan over medium heat, combine 1 1/4 cups sugar, cornstarch, flour, and salt. Gradually stir in cold water until smooth. Cook, stirring constantly, until mixture boils. Boil 1 minute, stirring constantly, then remove from heat.
In a small bowl, beat egg yolks with a fork. Stir about 1/4 cup of hot mixture into egg yolks. Gradually stir yolk mixture into hot mixture. Bring to a boil and continue to cook, stirring constantly, until thick. Remove from heat.