For the Starter: Dissolve yeast in warm water. Add 1 tablespoon caster sugar, 1 tablespoon vinegar, 1/2 teaspoon salt, 125g flour and 250ml milk in a non-metal bowl and stir with a non-metal spoon until creamy. Cover bowl with cling film and let stand in a warm place for two days to ferment. It will become bubbly and have a sour odour. Do not place in refrigerator. On second day, third day and fourth day: stir. On fifth day, add 125g flour, 200g sugar, 250ml milk and stir. On sixth, seventh, eighth and ninth day: stir. On tenth day add 125g flour, 200g sugar and 250ml milk. Reserve 250g starter; pour remaining starter, 250g each, into 3 containers to give away to friends if desired. This starter is enough for 8 loaves.
For the Cakes: Preheat oven to 180 C / Gas 4. Grease two 20 or 23cm loaf tins.
Mix 250g starter, oil, baking powder, eggs, sugar, flour, salt, vanilla and walnuts. Stir in the bicarbonate of soda just before pouring the batter into the prepared tins.
Bake at 180 C / Gas 4 for 45 minutes to 1 hour. Bananas, dates or raisins can be added for variety.