Preheat the oven to 180°C/350 ºF/Gas Mark 4. Line and grease an eight-inch round cake tin. Melt the butter and set it aside to cool.
Beat together the eggs, sugar and vanilla essence till light and creamy.
Gradually fold in the flour, stirring with a light hand only in one direction. Stir in the melted butter. Pour the batter into the prepared tin and bake for twenty-five to thirty minutes, till well risen and golden brown on top. Turn out onto a wire rack and leave to cool completely.