Place all ingredients except butter and oil in a mixer bowl and mix for 6 minutes at medium speed. Stop mixing, pick up what is left on the sides of the bowl and turn it back on at medium speed, then add 20 g butter at a time, until it is completely mixed into the dough. Continue mixing for another 6-8 minutes until the dough becomes smooth, shiny and elastic and begins to separate from the bowl.
Transfer the dough to a large bowl that you have previously floured, cover with a cloth, place in a warm place and let rise for 2-3 hours. Once the dough has doubled in size, knead it and leave it for another 4 hours or overnight in the fridge or another cool place.
This dough does not need to be rolled out and cut with a mould, but is shaped by hands into balls. Place the doughnuts onto a floured surface and let them rise once more for one to two hours. Fry them in deep oil heated to 190 °C for 2 min on each side. You can roll warm doughnuts in granulated sugar with some cinnamon or sprinkle with icing sugar when cooled.