Preheat the oven to 190C/Gas Mark 5. Line a muffin tin with muffin paper cases. I've used a large 6 hole muffin tin, but the recipe can be split between 12 normal muffins.
Mix together the flour, bicarbonate of soda, salt and half the mixed spice together. Add 6 tablespoons of the sugar and mix well.
Add the 3 egg whites to a clean bowl and lightly whisk the egg whites together with a fork. If using a mixer, use very low speed.
Add the softened butter, yoghurt and vanilla extract and beat together until blended.
Stir blackberries into the egg mixture until well mixed.
Carefully stir the egg mixture into the flour mixture until just blended. Do not over stir the mixture, it's absolutely fine if it's a little lumpy. In fact, if you overmix your mixture, the muffins will be very tough and rubbery.
Spoon the mixture evenly into the paper cases, filling each about two thirds full.
Mix the remaining sugar with the remaining mixed spice, then sprinkle it over the tops of the muffins.
Bake in the preheated oven for about 20 minutes or until well risen and springy to the touch.
Leave the muffins to cool slightly in the tin and then transfer to the wire rack to leave cool completely.