In the bowl of a kitchen mixer fitted with the dough hook attachment and add the flour, yeast, salt and caster sugar. Turn the machine on a low speed for 1 minute until all the dry ingredients are mixed together. Dice the butter straight from the fridge and mix together with the dry ingredients until fine breadcrumbs are formed.
Whisk the egg and milk together and pour into the bowl. Gradually increase the speed of the mixer until it is on full speed and leave to knead for a few minutes until the dough is stretchy. Turn off the machine, cover the bowl with a damp cloth and rest for 10 minutes.
After 10 minutes turn out the dough onto a floured surface and knead for 1-2 minutes before forming a ball and placing it back in the bowl. Cover again and rest for 1 hour or until doubled in size. Turn out onto the work surface and cut into 30g balls. Roll into balls and place onto an oiled tray. Oil a long sheet of cling film and cover the tray for the final prove.
Mix the caster sugar and ground cinnamon together, then place in a shallow pan. Set aside.
Deep fry at 180'C for 1 minute either side. Remove with a slotted spoon and immediately roll the doughnuts through the sugar.
Separate the egg yolks from the whites. Whisk the yolks with the sugar and cornflour until the mixture becomes a pale yellow colour.
Place the chocolate in a heat-proof bowl, and set aside.
Bring the milk to the boil then take off the heat and gently pour over the yolk mixture, all the while whisking to prevent any curdling.
Pour the mixture back into the saucepan and cook over a medium heat, stirring continuously until thickened. This will take no longer than 3 minutes.
Take off the heat and pour over the chocolate. Wait two minutes before stirring until smooth.
Transfer to a piping bag.
Make an incision in the end of each doughnut with a sharp knife. Insert the tip of the piping bag and fill the doughnut with the chocolate custard.
«Use a good quality chocolate with a cocoa mass percentage of 50% to 75%, depending on how intense a flavour you want.»