Sift the flour into a large bowl, then stir in the sugar and salt.
Whisk the egg, egg yolk and milk together in a jug. Make a well in the centre of the flour, then gradually beat the milk mixture into it with a wooden spoon, drawing in the flour from the edges to make a batter the consistency of single cream. Stir in the melted butter and vanilla extract.
Brush a 20cm non-stick frying pan with a little sunflower oil and place over a medium heat. When the pan is hot, pour in enough batter to coat the base of the pan evenly in a thin layer. Cook for about 1 minute, until light golden. Using a spatula, flip the pancake over and cook on the other side for a further 30 seconds, until golden. Set aside and keep warm, then repeat with the remaining batter, using a little more oil if necessary, until all the mixture is used up. You should have about 12 pancakes.
Serve the pancakes warm, sprinkled with sugar and lemon juice.