1: In a mixing bowl, cut 170 g butter into small pieces and leave at room temperature to soften.
2: Pour 110 g caster sugar and 10 g vanilla sugar into a medium-sized saucepan, then zest 2 lemons over. Mix in with a whisk, cover and leave the sugar to absorb all the flavour of the lemon zest (this can be prepared the day before and left overnight in the fridge, which is even better).