In a large bowl, mix the flour with the salt.
In a small bowl, activate the yeast with a little lukewarm milk and sugar (or use dried yeast directly in the flour). Let stand for 5 minutes.
Add the remaining milk, egg, and activated yeast to the flour. Mix until smooth, ensuring no lumps. The dough will be thick but still sticky.
Cover the bowl and let it prove in a warm place for about 60 minutes until the dough doubles in volume and bubbles start to form.
If using raisins/apples, gently fold them into the risen dough.
Heat the oil in a deep fryer or deep pot to 185 °C (365 °F).
Scoop the batter into the hot oil using two tablespoons (use one to scoop the batter and the other to push it into the oil) or use a small ice cream scoop.
Fry in small batches (3-4 pieces at a time) to prevent the oil temperature from dropping. The doughnuts should turn themselves over, but if they don't, assist them.
Cook for about 4-5 minutes until evenly golden brown on all sides.
Remove the finished Oliebollen with a slotted spoon and let them drain on a paper towel.
Serve them extremely hot. Just before serving, generously dust them with powdered sugar.
