For the doughnuts, put the flour, butter, eggs, yeast, sugar and milk in the bowl of a freestanding electric mixer and attach the dough hook. Knead on low speed for 8 minutes. Add the salt, then increase the speed to medium and mix for a further 5 minutes.
Transfer the dough to a lightly floured bowl, cover with a damp tea towel (dish towel) and leave the dough