Method for 10 apple rolls:
Dice the apples into small cubes, preferably with the peel. Sprinkle with lemon juice and leave to stand. Combine the flour, baking powder, sugar and salt. Add the softened butter and egg and mix until a crumbly dough forms. Add yoghurt and knead to a smooth dough. At this stage, the dough will be slightly sticky; add more flour only when shaping the rolls. Dust the dough well with flour, roll out into a rectangle and spread with softened butter. Combine the sugar, cinnamon and vanilla, put 1 tsp. aside and sprinkle the rest on the dough. Arrange the apple pieces on the dough and roll up lengthwise into a loose cylinder. Press down slightly and cut into 10-12 rolls. Place on a baking paper-lined tray. Preheat the oven to 200°C (top and bottom heat). Brush the pastries with whisked egg and milk, and sprinkle with sugar crystals if desired. Bake for 10-12 minutes, remove from oven and sprinkle with the remaining sugar mixture.
The rolls can be freshened up in the oven the following day: Sprinkle with a little water and bake for 2 - 3 minutes in a preheated oven (200°C, top and bottom heat).