Reserving a few blueberries for decoration, purée the blueberries in a blender or food processor. In a large saucepan, combine the blueberry purée with the lemon juice and vanilla essence. Cook over a medium heat for 5 minutes.
Combine the cornflour and 2 tablespoons water, and add to the fruit mixture, stirring constantly for about 3 minutes until thickened. Remove from the heat, and stir in the sugar.
Using the back of a spoon, press the mixture through a sieve to remove the blueberry skins. Layer in tall glasses, alternating with the vanilla yogurt. Decorate with reserved blueberries. Serve immediately or chill until needed.