Melt the white chocolate and butter together in the microwave. In my 1000 watt microwave this took 90 seconds. Stir until smooth, then stir in the lemon zest.
In another bowl, whisk the sugar and eggs together.
Add the melted chocolate mixture into the egg and sugar mixture, and whisk again.
Fold in the flour until just combined. Don't overmix.
Grease and line your slow cooker bowl. This was made in my 3.5 l slow cooker.
Pour the cake batter into the lined bowl.
Cook on high for 2-3 hours, or until a skewer or toothpick inserted in the centre comes out clean.
Note: all slow cookers are different, yours may take less time (or longer!), look out for burning and remove as soon as the centre is firm and the skewer or toothpick comes out clean.
Allow to cool.
To make the icing, whisk the crème fraîche with the icing sugar for 3-5 minutes, until soft peaks form. Stir in the lemon juice.
Spoon the icing on top of the cake and top with blueberries.
Keep the cake chilled. Best consumed on the day it is made.