Mix the butter and sugar until foamy, add the egg yolk and juice of one lemon, stir. Beat egg whites with a pinch of salt. Mix the flour with baking powder and fold into the butter mixture alternately with the egg white. Pour into a buttered mold dusted with flour.
Peel and halve the apples, cut out the core and cut into slices so that the halves remain whole. Turn in sugar and spread on the dough, press down a little. Bake in a preheated oven at 180 ° C for about 30 minutes