Combine remaining 1¼ c. sugar, cornstarch and salt. Add water; cook over low heat, stirring constantly, until mixture thickens and boils. Stir ½ c. hot mixture into egg yolks; stir this back into hot mixture. Return to heat and cook 1 minute, stirring constantly.
Remove from heat and add gelatin-sugar mixture. Stir until gelatin dissolves; stir in butter. Continue to stir until mixture starts to set but is still “soupy.” Add lemon juice and peel; fold in sour cream and nuts. Cool. Spread between layers of cake. Frost top and sides of cake with Lemon Fluff Frosting, tinted yellow.