Sieve the flour and baking powder together on the work surface and make a hollow in the centre. Add the egg yolk, vanilla, salt and lemon zest. Cut the butter into cubes and place around the edge. Quickly knead all these ingredients into a smooth shortcrust pastry. Leave to rest in a cool place.
For the filling, peel the apples, remove the cores and cut into thin slices. Mix well with the remaining ingredients. Roll out the pastry to approx. ½ cm thick and place on a baking tray covered with greaseproof paper. Spread out the apple filling over the pastry and fold in the edges over it. Brush the strudel with whisked egg yolk and bake in a preheated oven at 180º C for approx. 25 minutes. Sprinkle with icing sugar. The strudel tastes best lukewarm.