Mix butter, white sugar, eggs and rum together, add flour and mix well with a spoon.
Stir in sour cream.
Butter a 9" pie pan, placing half the mixture on the bottom. Sprinkle the bottom with the 2 tbsp shaved chocolate, add blueberries and icing sugar.
Cover with remaining mixture, sprinkle with remaining chocolate and bake for 1 hour and 15 minutes or until the crust is golden brown and a toothpick test comes out clean of batter, blueberries aside. Enjoy!
Recipe by This Lil Piglet at https://thislilpiglet.net/2016/07/blueberry-crumb-cake/