Mix flours, eggs, and water into a stiff dough and knead for 10 minutes until smooth.
Shape the dough into a ball and chill in the fridge for at least 30 minutes.
Flatten the butter into a 1cm thick rectangle and chill for one hour.
Roll the dough into a 1cm thick rectangle and place the butter over the bottom two-thirds.
Fold the top dough over the butter, trim exposed butter to layer on top, and fold the bottom half up.
Turn the pastry 90 degrees, roll into a rectangle, and perform a single turn by folding in thirds.
Chill for one hour and repeat the rolling and folding process twice more.
Chill the finished dough in the fridge overnight.
Preheat the oven to 200°C and grease a 12-hole muffin tin.
Cut 12 discs using a 9cm cutter and press them into the muffin tin.
Blind bake the pastry cases with baking beans for 10 to 15 minutes.
Whisk sugar, egg yolks, cornflour, and salt together in a bowl.
Heat milk with the vanilla pod and cinnamon stick until boiling, then strain.
Pour hot milk over the egg mixture, return to the pan, and cook until thick.
Melt the chocolate and mix with double cream to create a smooth ganache.
Spoon custard into cooled pastry cases and swirl in the chocolate ganache.
Bake the tarts for an additional 10 minutes at 200°C.
Cool the tarts and dust with icing sugar before serving.