Gently whisk together two thirds of the water, the yeast, egg yolks, oil, honey and vanilla extract in a large bowl.
Add the flour and salt and mix to combine, add the remaining water as needed to create a soft, slightly sticky dough. Knead for 5-10 minutes, either in a mixer or by hand until the dough is soft and supple. It should feel slightly tacky but not sticky.
Place the dough in a lightly oiled bowl, cover, and place in the refrigerator overnight. (It can be left in the fridge for a couple of days if needed)
The following day, take the dough out of the fridge and divide into as many equal portions as strands you are going to plait (I did 6, but you can just do a 3 strand plait if you want to keep it simple).
Drain the cherries, roughly chop them and then pat dry with kitchen roll, you don't need to get them completely dry but try to remove any excess liquid. Mix the cherries together with the chocolate, sugar, cornflour and cinnamon (if using).
Roll each piece of dough out into an approx 14x5 inch oblong (if you are doing a six strand plait), or 14x8 inch if you are doing a three strand one.
Divide the filling equally among the pieces of dough and spread it out over the centre of each, leaving an inch gap around the edges. Roll each piece up tightly from the long side (like making cinnamon rolls), pinch to seal, and then roll each cylinder (with your hands) into 18-21 inch rope-like strands, tapering the ends slightly.
Braid the strands together as you like, using the above video for help to make a six strand braid if needed.
Transfer the braid onto a baking sheet lined with greaseproof paper or a silicone liner, lightly cover with greased clingfilm and leave to rise for about 1 ½ hours until doubled in size.
Preheat the oven to 180C/350F/gas mark 4.
Brush the risen challah with egg wash and bake for 45-50 minutes. Give it a second coat of egg wash after 15-20 minutes and cover loosely with tin foil after about 30 minutes if it is getting too dark.
Slide the baked challah onto a wire rack and leave to cool completely before slicing.