Combine 3/4 cup sugar and cornstarch in a 2-quart heavy saucepan; stir well. Add buttermilk;cook over medium heat,stirring constantly, 6 to 8 minutes or until mixture comes to a boil and thickens. Stir in butter.
Combine egg yolks, lemon rind, and juice; beat well. Gradually stie one-fourth of hot mixture into yolks; add in remaining hot mixture, stirring constantly. Cook over medium heat, stirring constantly, 6 to 8 minutes or until snooth and thickened. Spoon mixture into four 10-ounce ungreased custard cups.
Beat egg whites in large bowl at high speed with an electric mixer until foamy. Gradually add 1/4 cup sugar, 1 tbls. at a time, beating until stiff peaks form. Spread meringue ove custards, sealing to edge of each dish.
Bake at 325F for 20 minutes or until meringues are browned. Cool completely before serving. Garnish if desired.