Dark chocolate and avocado cupcakes

Start by heating the oven to 180C/350F/Gas 4. Then break up the chocolate bars and melt gently in a bowl over simmering water. When melted, remove the bowl from the heat and leave to cool. Now put the butter and sugar into a food mixer and beat until light and creamy. Then beat in the eggs one at a time- beating well after each addition. Next stir in the sour cream on low speed, followed by the flour and finally the melted chocolate, mixing gently until thoroughly combined. Spoon the mixture into the muffin cases and bake for 25 minutes until firm to the touch. Remove the muffins from the tin and leave to cool on a wire rack. Meanwhile make the topping. Using a hand mixer, blend the avocado with the lemon juice until you achieve a smooth consistency, working out any lumps. Slowly blend in the sugar, increasing the speed of the mixer as you go (feel free to add more or less sugar to achieve a consistency & taste you like). Then spread the topping over the cooled cakes. Finally using a zester, create some dark chocolate shavings and sprinkle some onto the frosting and top with a cooled, toasted walnut.