Sift flour, baking soda and salt into a bowl and set aside.
Mix both types of sugar and the lemon zest in a food processor (Moulinette) for about 30 seconds. This way, the sugar absorbs the lemon flavor better and you won’t have such large pieces of zest in the cookies later. Transfer to a large bowl and mix with the softened butter until the mixture is whitish-creamy and the sugar is dissolved. Stir in the eggs, one at a time, and add the lemon juice. Finally, fold in the sifted flour in batches.
Using a tablespoon, spread the dough onto a baking sheet lined with parchment paper and flatten each cookie slightly to about 9 cm in diameter (saucer size). Leave enough space between the cookies.
Sprinkle with the vanilla sugar. I always have 6 cookies on a baking sheet, then the spacing is correct, as the cookies will get even bigger while baking. Bake the cookies in a preheated oven at 175°C for about 15 minutes until golden brown. Let them cool completely on a rack.