Heat oven to 190C/170C fan/gas 5. Brush 2 x 20cm cake tins with melted butter, and line the bases. Remember to dust well with flour. Prepare a pan of simmering water.
Place the sugar and eggs in a heatproof bowl, then pop over the simmering pan of water. Whisk around 5-6 minutes until the mixture has tripled in volume and is pale in colour. When you have removed from the heat you can gently fold in the melted butter.