Heat oven to 200C / Fan 180C/ Gas Mark 6 and butter a 12 hole muffin tin.
In a large bowl, combine the flour, sugar, baking powder, lemon zest and salt. In a separate bowl, mix together the buttermilk, eggs and butter.
Chop up any of the fruit which is too large into bite size pieces (e.g. strawberries). Make a well in the centre of the dry ingredients and pour in the buttermilk mixture. Stir until the ingredients are just combined and the mixture is quite stiff, but be careful not to overmix.
Add the lemon juice and carefully fold in the summer berries and the white chocolate chips. Spoon the mixture into the prepared tins, generously filling the holes.
Bake for 25 minutes or until risen and golden on top. Leave to cool in the tin for a few minutes, then run a palatte knife around the edge of the muffins and carefully transfer to a wire rack. Allow to cool completely.
Once cool, prepare the white chocolate butter icing: Put the butter into a bowl and beat until creamy.
Stir in the icing sugar gradually until a smooth, soft icing has formed, then mix in the melted white chocolate until well combined.
Spread the butter icing onto the top of each cold muffin or pipe on using a piping bag fitted with a star nozzle.
Sprinkle a small handful of the granola onto the top of each muffin
These muffins are best eaten within 1-2 days
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