1. To make the pastry, place the flour into a bowl, add the butter and sugar and with your fingertips rub until it resembles fine bread crumbs.
2. Add the lemon zest and stir through the mixture. Add the egg and yolk and mix together to form a soft dough. At first, the mixtures appears dry but as you work it the pastry comes together. Wrap in cling film and leave in the fridge to chill.
3. To make the filling, drain the ricotta through a sieve to remove any liquid. Pass the drained ricotta through a sieve a couple of times until smooth and creamy.
4. Mix with the sugar, chocolate chips, orange zest and vanilla extract.
5. Heat your oven to 180°C/Gas 4. Grease and line the base of a 20cm Tart Tatin tin with parchment paper. Grease the sides with butter. Take the pastry from the fridge and cut off a quarter to use as the lid.
6. Lightly dust your work surface and roll the pastry to 4-5mm thick. Line the prepared tin with the pastry trimming the edges to neaten. Sprinkle half the cake crumbs into the base. Cover the cake crumbs with the ricotta filling and spread out to level the surface.
7. Top the ricotta with a layer of the remaining crumbs. Roll out the remaining pastry just bigger than the tin. Place the tin on the rolled out pastry and cut round the base to make a lid just the right size.
8. Sit the lid on top. Brush the edges with egg white then fold the exposed sides of pastry down onto the crumb, this will be about 1cm.
9. Bake in the oven for 40-45 minutes until the pastry is golden and crisp.
10. Leave to cool in the tin for 10 minutes before running a knife around the edge and turning out onto a serving plate. Dust the top heavily with icing sugar.