1. Mix the flour, sugar, and salt in a large bowl. Add the vanilla extract as well as the cold butter in small pieces and mix everything with your fingers to get different-sized streusels. Transfer about 2/3 of the streusels to a 24cm tart tin with a removable bottom* and firmly press them into the bottom and up the sides. Place both, tart base and the remaining streusels in the fridge until needed again.

2. Squeeze out the lemons, collect the juice, and strain it through a sieve to remove any seeds and pulp. Combine about 11.8 fl. oz./350ml of juice with 5.0 fl. oz./150ml of water – a little less lemon juice and more water is fine if you prefer a less sour tart filling.

3. Mix the cornstarch with a little of the lemon juice and water mixture until smooth. Pour the remaining liquid into a small saucepan along with the sugar and butter. Bring the liquid to a boil – the butter should have melted completely. Stir in the cornstarch mixture while stirring constantly and let everything cook/bubble for a minute. The mixture should thicken considerably, like a custard. Remove from the heat and let it cool down a bit.

4. Whisk the egg and egg yolks in a small bowl, then add a spoonful of the lemon custard to the bowl while mixing constantly. Be careful not to overheat the egg, or it will curdle. Pour this mixture into the saucepan with the lemon custard while mixing constantly. Return the saucepan to the stove, bring everything to a boil again while mixing constantly, and cook the custard for another minute. Remove the from the heat and let the lemon tart filling cool down for about 10–15 minutes, stirring occasionally to prevent a skin from forming.

5. Meanwhile, preheat the oven to 180°C (350°F). Remove the tart tin and the streusels from the fridge. Pour the lemon filling into the streusel base and sprinkle the streusels evenly on top. Bake the tart in a preheated oven for about 38-40 minutes. Turn off the oven and leave the tart inside for another 15 minutes with the door closed. Remove the tart from the oven and let it cool down completely on a wire rack. Place the cooled tart in the fridge for at least 6 hours (or overnight). If desired, dust the tart with confectioners’ sugar and serve with whipped cream (optional).