In a mixing bowl, combine the milk, sugar, and custard powder. Stir on speed 2 at 90°C (194°F) for 6.5 minutes. Transfer to a bowl and cover the surface tightly with cling film.
For the dough, add all dough ingredients to the mixing bowl and knead for 2 minutes. Pour into a bowl and let rise in a warm place for 30 minutes.
While the dough rises, peel and core the apple, then cut into small pieces. Stir 70 g sour cream into the prepared cream.
Roll out the yeast dough into a 30 × 40 cm rectangle. Spread the cream generously over it, leaving a 2–3 cm border. Distribute the apple pieces over the cream. Brush the border with 10 g sour cream. Fold the long sides inward, then roll tightly from one of the longer sides. Using a large serrated knife, cut into 2–3 cm thick snails, yielding approximately 15 pieces.
Place the snails on two baking sheets lined with baking paper. Let rise for 20 minutes. Preheat oven to 175°C (347°F). Bake for approximately 15 minutes until golden brown. Dust with icing sugar while still warm.