Warm milk to 120C.
Add sourdough starter, flour, salt, sugar, cinnamon (if using) to a bowl. Stir in milk, egg, butter using a table knife until thoroughly combined and dough comes together.
Knead dough on a lightly floured surface for 10-15 minutes. Form a ball and pop into a lightly greased bowl. Cover and set aside for 1 hour at room temperature to rise.
Pull corners into the middle of the dough. Turn over, cover and set aside. Repeat each hour several times. Finally, repeat and proof overnight.
In the morning, roll out onto a lightly floured surface to 3cm thick. Cut out using a large circle cutter. Use a small cutter to cut out middle.
Put onto a lined baking tray and brush with oil. Cover with greased foil and set aside for 30 minutes to one hour to rise.
Preheat the oven to 170C. Melt 30g butter and brush over dough. Flick water onto the tray between the doughnuts. When it evaporates it creates steam which helps cook and rise doughnuts further. Cook for 10-15 minutes until golden.
Melt 30g butter. Dip into the melted butter whilst warm and dip mmediately into sugar or cinnamon sugar. Alternatively cool and ice with icing sugar mixed with a few drops of water then add sprinkles. For glazed doughnuts, whilst warm mix together icing sugar and water to create a running consistency and dip in doughnut whilst warm. Set aside on a cooling tray to dry.
If you wish to freeze them, you can do so. Freeze before dipping in butter and sugar or icing and do this once defrosted.
