To make the frosting:
Pre-heat oven to 200˚C/Gas Mark 6.
Melt butter in Bain-marie and set aside to cool a little.
Place dry ingredients into a mixing bowl. (Top tip: Do not sieve dry ingredients as it gives a better texture to the sponge)
Mix butter, eggs and milk into a separate mixing bowl and add to the dry ingredients using a hand mixer. Don’t over whisk, the lumpier the mixture, the better the texture of the sponge.
Gently fold the blueberries and raspberries into the mixture and then spoon into 12 muffin cases. Place in the oven for 20 minutes or until fully risen and golden brown.
Leave to cool on wired rack.
To make a natural blueberry food colouring for the frosting, place the blueberries and sugar with 2tbsp of water into a sauce pan and heat gently until the sugar has dissolved. This should take about 5 minutes. The contents should start to bubble a little and the juices from the blueberries should be seen. Take the sauce pan off the heat and run the blueberry mixture through a sieve leaving you with a smooth rich food colouring. Leave the colouring to cool. The blueberries in the sieve can be thrown away.
Using a hand mixer, whisk together the icing sugar and butter until smooth and glossy.
Gently fold the food colouring into the frosting. The amount you use is personal preference.
Place frosting into a piping bag with a round nozzle and pipe smooth thick swirls on top of the sponge.