Preheat oven to 180°C. Grease and line a 3cm deep, 24cm x 30cm (base) lamington pan. Using an electric mixer, beat butter and ½ cup sugar until pale and creamy. Stir in egg, cornflakes, coconut, flour, cocoa and a pinch of salt.
Press mixture into pan. Bake for 20 minutes or until light golden. Cool.
Bring pineapple and juice to the boil in a saucepan over medium heat. Add jelly crystals and remaining ¾ cup sugar. Cook, stirring, until dissolved. Transfer to a bowl. Refrigerate for 1 hour or until almost set.
Beat milk and lemon juice until thickened. Fold into jelly mixture. Pour filling onto cooled base. Refrigerate for 2 to 3 hours or until firm. Serve with ice-cream.
