Shortcrust: Combine the butter with the icing sugar, vanilla and salt in a bowl then beat until the mixture is light and creamy. Add the flour then knead with the yolk and water into a smooth dough. Adjust the water so that the dough is neither too wet nor too dry.
Filling: Spread the butter on the bottom and the edges of a tart tin. Sprinkle the sugar.
Peal the apples and pears then core and cut into 2cm thick pieces. In the tart tin lay the apple in a star shape with the pieces close together. Sprinkle with the lemon juice.
To Assemble: Roll out short pastry circle in the size of the tart tin then place it over the pears and apples. The overhanging dough at the edge of the mold press firmly so that the fruit is tightly enclosed. Use a fork to pierce many air holes in the dough.
Bake in preheated oven for about 10 minutes. Drop the temperature to 180 degrees C then bake for another 35 minutes until the shortbread is golden brown.
Switch off the oven, open the door and let the tart stand for about 3 minutes in it.
Remove the tart from the oven, place a large plate over it and then quickly turn over. Do not wait too long before doing this step otherwise the apple and its caramel may stick to the tart tin.