Drain pineapple very well.
Measure juice and add water if necessary to make 1 cup.
Combine cornstarch, sugar and salt in saucepan.
Add the 1 cup pineapple juice gradually, stirring and blending until smooth.
Set over moderate heat and cook, stirring constantly, until mixture boils and is thick and clear.
Remove from heat and stir in pineapple, lemon juice and butter.
Cool thoroughly before filling cake.