Line a 12-hole muffin tin with squares of greaseproof paper. Split the vanilla pod lengthways, using the tip of a sharp knife, and place in a small saucepan with 100 ml (3½ fl oz) of the milk. Bring just to the boil then remove from the heat and leave to cool slightly. Remove the vanilla pod from the pan and scoop out the seeds with a teaspoon. Stir them into the milk and discard the pod.
Sift the flour and baking powder into a large bowl, then stir in the sugar. In a separate bowl, beat together the eggs, vegetable oil, yogurt, vanilla milk and remaining milk. Using a large metal spoon, gently stir the liquid into the flour until only just combined.
Divide the mixture between the muffin cases and bake in a preheated oven, 200°C (400°F), Gas Mark 6, for about 20 minutes, until well risen and golden. Transfer to a wire rack and dust with icing sugar. Serve slightly warm.