Grease a tart mould (without lifting base) with a rim of approx. 4 cm high (26 cm Ø). Sprinkle the bottom of the mould with brown sugar and spread 25 g butter in flakes on top.
Wash lemon hot, dab dry. Finely grate the peel. Squeeze lemon. Peel apples. Core with an apple corer and cut in half crosswise. Cut round apple sides more often about 1 cm deep.
Drizzle apples with 2 tbsp. lemon juice. Place in the mould with the round side down. Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer).
For the dough, mix 150 g butter, sugar, lemon zest and 1 pinch of salt until creamy. Stir in the eggs one by one. Mix flour and baking powder and stir in alternately with yoghurt and the rest lemon juice.
Put the dough on the apples. Carefully smooth it down. Bake in the hot oven on the lower shelf for 45-60 minutes (test with chopsticks!).
Let the cake cool down for about 5 minutes on a cake rack. Remove from the edge with a knife. Carefully turn out onto a cake plate and let it cool down. Dust with icing sugar.