Cream butter and castor sugar in an electric mixer until pale and creamy, about 10 minutes. Fold in unbeaten egg whites a little at a time. Mix in flour until smooth. Refrigerate dough for 30 minutes.
Using a palette knife, spread a thin layer of wafer mixture over a buttered and floured 30 cm x 24 cm baking tray, then sprinkle evenly with shredded coconut. Refrigerate until firm, about