Heat the oven to Gas Mark 4, 180°C, fan 160°C. Line a 12-hole muffin tin with cases.
Put the flour and baking powder into a mixing bowl, add the butter and rub into the flour until it resembles breadcrumbs. Stir in the sugar and hazelnuts. Mix the eggs and the yogurt together and stir into the mixture with half the blackberries.
Divide the mixture between the muffin cases and push the remaining berries into the top of each. Sprinkle over the reserved nuts and demerara sugar. Bake for 20 minutes until well risen and golden brown.