Pre-heat the oven to 225°C/fan 200°C. Butter two 12-cup muffin tins and line with paper liners.
In a bowl, combine the flour, baking powder, and baking soda using a whisk. And set aside.
In another bowl, whisk together the buttermilk, sour cream, eggs, caster sugar, vanilla extract, salt, and oil until smooth. Add the melted butter and whisk well.
Now add the flour mixture to the wet ingredients - making sure to reserve 2 tbsp flour mixture to later coat the blueberries.
Mix using a silicon spatula until barely combined and the batter still has flour spots.
Add the blueberries to the remaining flour and shake around to coat. Then fold delicately into the batter.
Spoon batter into the prepared muffin tins, filling right to the top. Sprinkle generously with demerara sugar.
Bake in the pre-heated oven for 10 minutes, then reduce the temperature to 175°C/fan 160°C and bake for a further 15-16 minutes, or until a skewer inserted into the centre of a muffin comes out clean.
Allow to cool down and store in an air-tight container for up to 3 days. Or freeze for up to a month.