Grease a tart mould (26 cm Ø) with a removable base, sprinkle with breadcrumbs. Knead flour, 175 g sugar, vanilla sugar, 1 pinch of salt and 275 g butter in flakes first with a hand mixer, then briefly with your hands to form crumbles.
Finally, add 1 tablespoon of sugar. Approx. 2⁄3 Press the crumble firmly into the mould as the base and rim (approx. 2 cm high). Chill the rest of the crumbles.
Squeeze the lemons, remove 2 tablespoons of lemon juice. Fill the rest of the juice with water to 450 ml, bring to the boil with 175 g sugar and 15 g butter. Mix starch and 4 tbsp. water. Stir into the boiling juice, simmer for about 1 minute while stirring.
Whisk the egg yolks. Gradually add 3 tbsp. lemon cream to the egg yolk, then stir into the rest of the cream. Bring the cream to the boil again briefly, then pass through a fine sieve and allow to cool for about 15 minutes.
Fill the cream into the tart tin. Sprinkle with the remaining crumbles, rubbing the crumble dough between your fingers. Place the tart in the preheated oven (electric oven: 175 °C/circulating air: 150 °C/gas: level 2) on the lowest shelf (gas: see
oven manufacturer) bake for 25-35 minutes. Leave to cool and rest in the mould for at least 4 hours to allow the cream to set (it is then soft but does not melt!).
Mix icing sugar and 2 tablespoons of juice until smooth. Pour the icing into a freezer bag, cut off a small corner at the bottom. Distribute in strips on the tart. Let it dry.