Preheat the oven to 180°C, fan 160°C, gas 4. Grease 2 x 18cm round cake tins with oil, then line the base of each with baking parchment.
Cream the butter and sugar together until light and fluffy. Gradually add the eggs, a little at a time, beating well between each addition.
Fold in the flour and add 3 tablespoons water to bring the mixture to a dropping consistency. Divide the mixture between the 2 tins and bake on the middle shelf for 25 minutes, until well risen and golden.
Leave the cakes to cool slightly in their tins, then remove the base liners and transfer to a wire rack to cool completely.
Cook’s tip: when baking sponge cakes make sure you remove any baking trays that normally live in your oven. That way the air can circulate properly.