Mix the flour, sugar, baking powder, baking soda, 1/2 teaspoon lemon crust, cinnamon and salt. Separatly mix the yogurt, oil, eggs and 1/2 teaspoon lemon crust. Add to the dry mixture and stirred carefully. Finally add the blackberries and walnuts. Pour into loaf pan.
Mix the 1/3 sugar with the 1/4 flour and the 30g butter. Spread over the muffins.
Bake for about 20-25 minutes in advance oven (180 degrees). Cool bread in pan then remove and place on a plate to cool completely.