Grease a box mould (approx. 1 1⁄4 l capacity; approx. 26 cm length) and dust with flour. Wash 1 lemon hot, dab dry and finely grate the peel. Squeeze 2 lemons. Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see
Mix butter and 200 g sugar with the whisk of the mixer for about 5 minutes until white and creamy. Stir in the eggs one by one. Mix flour, baking powder, lemon zest, vanilla sugar and 1 pinch of salt and stir in alternately with the milk.
Stir in 3 tablespoons of lemon juice. 2⁄3 of the blueberries under the dough and fill into the mould. Sprinkle with the remaining berries. Bake in the oven for about 1 hour, cover with aluminium foil about 15 minutes before the end of the baking time.
For the rest of the syrup, boil lemon juice and 75 g sugar. Remove the cake from the oven. Prick several times while hot with a wooden skewer and sprinkle with lemon syrup. Let the cake cool in the mould for about 30 minutes.
Then fall down and let it cool down.