Preheat an oven to 200 degrees C. Grease 12 cups of a muffin tray or line with paper baking cups. Whisk the milk, egg, butter and vanilla essence together in a bowl.
Mix the flour, oats, brown sugar, baking powder, bicarb soda, mixed spice and salt in a bowl. Make a well in the centre and pour in the egg mixture. Stir until just combined. Gently fold in the blackberries. Divide the batter evenly into the muffin cups and sprinkle with the sugar.
Bake in the preheated oven until a toothpick inserted into the centre of a muffin comes out clean, about 20 minutes. Cool in the tray for 10 minutes before removing to cool completely on a wire rack.
Also a great way to use up extra berries. Just throw in what you have you don't need to limit yourself to blackberries. Currants, raspberries or even cut-up strawberries work fine. The brown sugar can be reduced or even omitted if you prefer a less sweet muffin.