Preheat the oven to 200°C, fan 180°C, gas 6. Line a 12-hole muffin tin with paper cases.
Sift the flour and baking powder into a large bowl. Stir in the sugar.
In a separate bowl, combine the melted butter with the eggs and yogurt.
Gently stir the liquid mixture into the flour mixture, gradually incorporating all the flour. Fold in the blackberries and blueberries. Spoon the mixture into the muffin cases, filling nearly to the top, then bake in the oven for 25 minutes.
Transfer to a wire rack and leave to cool.
Cook's tip: if berries aren't in season you can always use frozen ones.