Preheat oven to 180 deg C. Grease a round 24cm cake tin and dust with flour.
Mix the 5 egg yolks with 100g of the sugar in a heatproof bowl. Place the bowl over a pan of barely simmering water, ensuring the bowl doesn't touch the water. Whisk until the mixture is lukewarm. Remove bowl from heat and whisk until completely cooled.
In a separate bowl with a clean whisk, combine the egg whites with a pinch of salt and beat to soft peaks, then beat in the remaining 50g sugar and whisk till stiff peaks form.
Gently fold flour and melted butter into the cooled yolk mixture. Once smooth, fold in the egg whites. Pour mixture into prepared tin.
Bake for 30 minutes in the preheated oven. Remove from oven and let cool completely.