Put butter with 25 g sugar, rind of 1/2 lemon, salt and flour in a bowl. Knead first with the dough hooks of the hand mixer, then with your hands to a smooth dough. Grease a tightly closing, square springform pan (24 x 24 cm). Pour in the dough and press it with your hands to a flat bottom. Chill for about 30 minutes.
Bake in the preheated oven (electric range: 175 °C/ convection oven: 150 °C/ gas: level 2) for 20-25 minutes until golden brown. Let cool off. In the meantime, whisk eggs and 200 g sugar with the whisk of the hand mixer for about 5 minutes until thick and creamy. Stir in double cream. Stir lemon juice and the rest of the grated zest into the egg cream.
Pour lemon cream on the bottom. Bake in the oven at the same temperature for 20-25 minutes. Let it cool down on a cake rack. Cut the cake into about 24 pieces and dust with icing sugar
waiting time approx. 1 1/2 hours