Mix yeast packet with the warm water and sugar, and let the foam double in size.
Mix the flour, sugar, water, activated yeast, lemon zest, salt and eggs into the bowl of an electric mixer with a dough hook attachment and mix on medium speed for 1-2 minutes, until it's all combined. Then turn up the speed to medium high and let it run for about 5 minutes, or until the dough starts coming away from the sides and forms a ball. Turn off the mixer and let the dough rest for a minute. Slowly add the butter to the dough, about 25g at a time. Once it is all incorporated, mix on high speed for 5min until the dough is glossy, smooth and very elastic when pulled.
Cover the bowl with saran wrap and leave to proof in the bowl until it has doubled in size - about 1-2 hours. Punch down the dough – then re-cover the bowl and put into the fridge to chill overnight. The next day, take the dough out of the fridge and cut it into equal, palm size pieces (you should get about 18). Roll them into smooth, taut, tight buns and place them on a floured baking tray, leaving plenty of room for them – so they don't stick together while they proof. Cover lightly with Saran wrap and leave out in a 75F room for about 4hr, or until about doubled in size.
Frying and finishing
In a big cast iron pot, prepare the deep fryer; fill it up to the halfway point with the oil. Heat the oil to 350F, then carefully remove the doughnuts from the tray by sliding a floured pastry scraper or fish spatula underneath them, taking care not to deflate them, and put them into the oil. Don’t overcrowd the fryer – do two to three per batch, depending on the size of your pan.
Remove from the oil and place on a paper towel, then toss them in a bowl of caster sugar while still warm. Set aside to cool before filling. To fill the donuts, make a hole in the white strip in between the brown tops/bottoms. Fill a piping bag with jam (or any other filling you see fit!) and pipe into the doughnut until a little bit peeks out on top.
I've included volume ingredients here but BAKING TURNS OUT BETTER WHEN YOU MEASURE IN WEIGHT