Pre-heat the oven to 175°C (350°F). Grease and flour a small 18cm (7") springform pan.
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 stand mixer) whisk together the eggs and sugar.
Add the lemon juice, vanilla extract, melted butter, and melted chocolate, then whisk to combine.
Whisk in the flour and mix until you have a fairly smooth batter.
Lastly, stir in the blueberries.
Bake for around 18-27 minutes, depending on how molten you would like it (see note 2).
Remove from the oven and let the cake cool for a while before removing the outer part of the springform pan (see note 3).
Enjoy slightly warm or wait until the cake has cooled completely. Sprinkle with a little icing sugar before serving and garnish with a sprig of fresh mint, if desired.