In a food processor, sand dry ingredients and butter butter first. Add the yolk, milk and vanilla and mix to combine. Wrap up and chill completely. Grease tart pan with softened butter. Flour work surface and work out dough to soften – the dough should remain cool and supple. Roll out shortcrust into a 3 mm thick disk. Lay over the tart pan and shape into tart shell.
Blend all ingredients together and chill.
Fill the tart shell with the room temperature apples mixture. Cover with the custard two-third up (apples should remain visible like icebergs).
Beat egg whites with sugar, and add melted butter. Set aside.
Bake at 350ºF/180ºC for 30 minutes. Remove tart from the oven, pour over tart glaze and bake for an additional 10 minutes at 425ºF/220ºC. Serve at room temp – Enjoy!