whisk the milk, water and yeast together in a bowl. 
pulse the cold butter and flours in a food processor or rub it between your fingers until it resembles breadcrumbs... but still has a few lumps. 
stir the sugar and salt into the flour mix and ti everything into the wet bowl. 
stir together a little, then get your hands involved to bring it into a dough. 
cover loosely with clingfilm and leave it to rest for 2 hours. 
roll the rested dough out on a floured surface to about 1cm thick, aiming for a rectangle that is twice as long as it is wide. 
fold the dough into thirds so that the last third overlaps the other two. 
turn the dough through 90 degrees and repeat the process twice more. 
cover with clingfilm and leave in the fridge to rest for at least 4 hours, overnight is better. 
roll the dough out once more, to the thickness of a pound coin (1/2 cm) then use cutters to shape your doughnuts. 
lay them on a floured tray and cover with a clean tea towel at room temperature to rise for another hour. 
fry them, a few at a time, in a few centimetres of oil heated to 170ºC for 2 minutes on each side until puffed up and golden. 
drain on kitchen roll briefly then toss in vanilla sugar before leaving to cool. 
inject the vanilla custard into the cronuts at 5 or 6 points around the ring using a piping bag with small nozzle or syringe.
mix enough lemon juice with the icing sugar to create a glacé icing and colour them (optional) to your preference. 
drizzle over the cooled and filled cronuts, allow to set and tuck in!