Preheat the oven to 200C and line 3 baking trays that are at least 24cm x 30cm (it doesn't matter if they're a little bit bigger as you will trim the finished cake).
Put the sifted icing sugar, ground almonds and sifted flour in a large mixing bowl, add the eggs and beat until the mixture is light and pale.
Melt the butter gently, do not let it boil or simmer, add the melted butter to the egg mixture and beat until it is fully mixed in.
Put the egg whites and caster sugar in the bowl of a stand mixer and whisk until at firm peaks stage.
Fold the meringue into the egg mixture until fully mixed, weigh the cake mixture (the easiest way to do this is to tip the mixture back into the bowl of the mixer and weigh it) and then split it between the 3 baking sheets.
Bake in the preheated oven for 12-15 minutes until golden brown and the sponge springs back when pressed gently.