Preheat your oven to 180⁰C/Fan 160⁰C/Gas Mark 4 then add muffin cases to a 12 cup muffin tin.
Roughly chop the walnut pieces then set aside.
Beat the eggs, brown sugar and oil together in a bowl then stir in 4 tablespoons of the orange juice.
Sieve the flour, baking powder and cinnamon into the bowl then fold until evenly combined.
Stir in the carrot, sultanas and 85g of the walnuts, leaving 15g to decorate.
Spoon mixture between the cases then bake in the centre of your preheated oven for 30 minutes then transfer to a wire rack to cool.
Meanwhile beat the cream cheese, butter, icing sugar and orange zest together until smooth. Using the back of a teaspoon to swirl the frosting over the top of the muffins, decorate with the remaining walnut pieces then serve and enjoy!