Pre-heat the oven to 150ºC (300ºF). Grease a small spring-form pan (mine is 6" / 18cm)
Cream the butter and sugar together until light and fluffy. Add the lemon juice and continue to mix until everything is evenly combined.
In a separate bowl, mix the flour, vanilla sugar and baking powder and then sift into the wet ingredients. Stir until everything is combined.
Pour the batter into the prepared pan and cook for around 20 minutes. The cookie will appear underdone but it is ready. When you remove it from the oven it will rapidly deflate but that's what you want to happen. Leave to cool completely.