Pre-heat the oven to 175°C (350°F). Grease and flour a small 18cm (6") springform.
Melt the chocolate and then set to one side.
In a separate bowl, melt the butter and again, set to one side.
In a third larger bowl (or using a mixer) beat the eggs and sugar together. Add the lemon, vanilla, butter and chocolate.
Add the flour together and mix until you have a fairly smooth batter. Stir in the blueberries.
Bake for around 18-27 minutes, depending on how well-cooked you would like it. For a fairly molten middle, the sides should be firm to the touch but the centre will still be a little jiggly when you gently shake the pan. I baked the one in the photos for 23 minutes.
Remove from the oven and allow the cake to cool for a while before removing the outside part of the springform (leave it on the base of the pan and don't attempt to transfer it to another dish, unless you have baked the cake right through and it is firm). Eat slightly warm or wait until the cake has cooled completely. Sprinkle with a little icing sugar before serving. Enjoy!