1. In a large mixing bowl, dissolve the yeast in the lukewarm milk. Let it sit for about 5 minutes until it becomes frothy.
2. Add the sugar, beaten eggs, and salt to the yeast mixture. Mix well.
3. Gradually add the flour, stirring continuously until you get a smooth batter. If using raisins, fold them into the batter.
4. Cover the bowl with a clean cloth and let the batter rise in a warm place for about an hour, or until it has doubled in size.
5. Heat the vegetable oil in a deep frying pan to 350°F (175°C).
6. Using two spoons, drop small balls of batter into the hot oil. Fry until golden brown, about 4-5 minutes per batch.
7. Remove the oliebollen with a slotted spoon and drain on paper towels.
8. Dust with powdered sugar before serving.