Soften gelatine in cold water.
Beat egg yolks and mix with frozen lemonade in a heavy saucepan.
Cook, stirring constantly, about 3 minutes, until mixture will coat a spoon.
Stir in gelatine mixture, mix well, and let cool for 30 minutes.
Beat egg whites until nearly stiff; beat in icing sugar until shiny stiff peaks form.
Fold into cooled custard mixture.
Cut angel food cake into cubes.
Alternate layers of cake and custard in two loaf pans or 1 angel food pan, beginning with cake and ending with custard.
Press out any air pockets.
Wrap and freeze for up to 3 months.
Cut off icy slices as required.