Preheat the oven to 180ºC. Lightly butter a 20cm square cake tin and line the base with baking paper.
Beat the eggs with the salt until very foamy, this will take about one minute with an electric beater. Add the sugar gradually, beating very well, and keep beating for about 5 minutes until the mixture is very pale and fluffy.
Sift together the flour and baking powder, then sift them again onto the egg mixture. Fold the flour gently into the egg and lastly fold through the melted butter.
When everything is well combined, pour the mixture into the prepared tin and bake for 25 to 30 minutes until the sponge cake is golden, well risen and springs back when lightly touched.