Beat in the vanilla and the lemon zest, lemon juice ... Using a skewer, poke deep holes all over the top of the cake. Brush the glaze all over the cake in several layers until thickly glazed.
As soon as the cake is cooked, pour the glaze over the top, leave to cool. If you want to ice the cake, mix equal parts icing sugar and lemon juice and spread over the cake, leaving to firm up ...
Let the cake cool completely on a wire rack. To make the glaze: Mix together the icing sugar and lemon juice to get a very thick pourable glaze. If it is too thin, add a little more icing sugar.