Beat together the butter, two sugars, egg and vanilla extract till light and fluffy.
75 g butter, 37.5 g light muscovado sugar, 50 g caster sugar (superfine sugar), 1 medium free-range egg, ½ tbsp vanilla extract
In a different bowl stir together the flour, baking powder and salt.
170 g plain flour (all-purpose flour), ¼ tsp baking powder, ⅛ tsp salt
Next stir the flour mixture into the butter mixture. I kneaded it together till it all came together like biscuit dough. Sides of the bowl should be clean when it is the right consistency.
Stir in the white chocolate and blueberries and make sure they are evenly distributed.
50 g white chocolate, 50 g blueberries
Grease your slow cooker bowl with butter, and cut grease proof paper to fit the base and put it in. Grease the top of the paper too.
Spoon the dough into the slow cooker bowl and flatten/press into the sides.
Slow cook for 3 hrs on low, with lid on. You may need to switch to high for the last 30 mins.
For the last 30 mins leave the lid half open.
Take the bowl out the slow cooker and cool for half an hour.Turn the cookie out on to a cooling rack, remove the grease proof paper for the bottom, and slice to serve.