Mix the flour, baking powder, sugar, vanilla sugar and lemon zest in a bowl. Add the butter and rub together using your hands to form an even, crumbly mixture. Add the eggs, mix quickly to form a soft dough – do not knead. Flatten the dough, cover and leave to chill for approx. 1 hr.
Mix the apples, sugar, cinnamon and lemon juice.
Halve the pastry. On a lightly floured surface, roll out one half to approx. 4 mm thick, place in the prepared tin. Press the dough in well around the edges and prick the base firmly with a fork. Spread the filling over the pastry base. On a lightly floured surface, roll out the remainder of the dough to approx. 4 mm thick, cut into 6 strips (each approx. 6 cm wide), lay on top of the filling. Press down firmly at the edges, brush with egg yolk.
Approx. 45 mins. in the lower half of an oven preheated to 180°C. Remove from the oven, allow to cool slightly, remove the tin frame, slide the pie onto a cooling rack and leave to cool completely.