Put the flour, caster sugar, salt, yeast, eggs and water into the bowl of a stand mixer fitted with the dough hook. Mix on medium speed for around 10 minutes, until the dough comes away from the sides of the bowl and wraps around the dough hook.
Slowly add the butter in small lumps with the mixer on medium, don't add the next batch of butter until the previous amount has been completely mixed in.
Once all the butter is added, mix on high for 5 minutes, until the dough is glossy, smooth and very elastic when pulled. Cover the bowl with clingfilm and leave to prove somewhere warm for two hours or until it has doubled in size. Knock back the dough, then re-cover the bowl and put into the fridge to chill overnight.
Next make the custard for the chocolate cream, put the milk in a saucepan, split open the vanilla pod and scrape out the seeds.Place the pod and the seeds in with the milk and warm them to just come to a boil.
In another bowl whisk the egg yolks, sugar and plain flour together until pale in colour.
Add the chopped chocolate to the hot milk and stir until the chocolate has completely melted. Pour the hot chocolate milk (return to the heat briefly if necessary) into the egg mixture and whisk the whole time to combine and make sure the eggs don't cook. Return the mixture to the saucepan and stir or whisk until the custard is very thick, this will take around 5 minutes.
Pass the custard through a sieve into a bowl and cover with cling-film, pressing it on to the surface of the custard and place it in the fridge to cool.
The next day sprinkle two baking trays lightly with flour and taking the dough cut off 50g pieces of dough and shape them into a tight ball. Place them on the baking trays spaced well apart with enough room to double in size. Cover with clingfilm and leave to prove for 4 hours or until doubled in size. You should end up with 20 doughnuts.
Get a large heavy-based saucepan and fill it up to the halfway point with the oil and heat the oil to 180° C. When the oil isready, gently pick up the doughnuts, trying not to drag them or to distort the shape too much. You will probably want to fry just one doughnut first, to check the oil and to get an idea of timings, about 2 minutes on each side should be enough. Remove the doughnut from the oil with a slotted spoon and place on a cake rack lined with paper kitchen towel.
After the first doughnut, you should then be able to fry two doughnuts at a time, but make sure the oil is always back to 180C before frying. If the oil is too hot, the doughnut will be raw in middle and burnt on the outside and if it is too cold, the doughnuts will be greasy. Continue until all of the doughnuts have been fried.
While the doughnuts are still warm, place the caster sugar in a bowl and roll the doughnuts in the sugar to coat them.
Next finish the chocolate cream, place the double cream in a large bowl and whisk until soft peaks. Take the chocolate custard from the fridge and stir to loosen it and break up any lumps. Gently fold in the whipped cream and place the custard in a piping bag fitted with a small round tip nozzle and put it back in the fridge to firm up a little.
When the doughnuts are cool pierce a hole in the side of the doughnut with a knife and fill with the chocolate cream (around 45g per doughnut) until the doughnut begins to swell. Then top the doughnut with a small blob of cream to hide the hole and top with a piece of chocolate if you wish.
Store the finished doughnuts in an airtight container, somewhere cool and consume within 48 hrs.
