Mix cornstarch, sugar & salt in top of double boiler. Gradually stir in water or orange juice, lemon juice & rind and butter. Cook over boiling water about 5 min, stirring constantly. Cover & cook gently 10 min longer without stirring. Remove from heat & stir egg yolks in gently. Return to heat & cook 2 min longer, stirring gently & constantly. Remove from heat & stir gently t till cool.