Make a simple, enriched dough. Put the flour in a large mixing bowl and stir the salt and sugar into it, then add the yeast.
Put the milk and butter in a saucepan over a medium heat and warm up until you can just hold your finger in the mik without needing to scratch it back quickly.
Mix well to melt the butter, then add the eggs and beat them in. slowly add the liquid ingredients to the dry ingredients, mixing them together with a dough scraper or a wooden spoon.
When the liquid has been incorporated, tip the contents of the bowl onto the worktop and knead for 10 minutes the mixture will be extremely sticky, so it’s best to knead it by picking it up and slamming it onto the countertop, then folding it over, this is why a dough scraper is such a great tool.
If you have a free standing electric mixer with a dough hook attachment, it might be easier to knead the dough in that for five minutes on a low setting. When the dough is smooth and elastic, place it in a floured bowl and cover with a damp cloth. Leave it to rise for one hour.
After the dough has risen, weigh it into 10 equal pieces.
Roll these pieces into perfect balls, place on a floured baking sheet or board well spaced apart and cover in cling film to rise again for 1 – 2 hours, or until doubled in size.
For the crème patissiere, whisk together the egg yolks and sugar in a bowl until it becomes light and thick. Stir in the cornflour. Bring the milk up to a simmer and then pour half of it over the egg mix, whisking it all the time.
Return the mix to the saucepan with the rest of the milk and stir it over a low to medium heat until it comes to a gentle boil. Continue to cook it, stirring constantly for around 2 minutes, or until it becomes thick.
Remove it from the heat. Cover it with cling film, making sure the cling film is placed directly on top to stop a skin forming and allow it to cool.
Heat the jam until it becomes runny and sieve it into a bowl to remove any bits. Pour the caster sugar on a plate and place it next to the deep fat fryer, alongside a cooling rack. Fifteen minutes before the dough balls are ready, heat the deep fat fryer to 180°C.
If you don’t have a thermometer, test the heat of the oil by dropping a cube of bread into it, if it sizzles then the oil is ready.
Fry the doughnuts two or three at a time, for about three minutes per side. Remove it from the fryer using a slotted spoon, dip it in the sugar and coat it well, then place it onto the cooling rack.
Put the jam into a syringe and inject each doughnut with a little bit, try and get it in the centre. Now repeat the process with the crème patissiere. If the jam or crème patissiere starts to ooze out, simply dip the hole into the caster sugar and stand the doughnut up so the hole is on top.