Line a 12-section muffin tray with paper muffin cases. Put the butter, muscovado sugar, eggs, flour, baking powder, mixed spice, ground walnuts and orange rind in a bowl and beat with a hand-held electric whisk for about a minute until light and creamy.
Stir in the grated carrots and sultanas until evenly mixed. Divide the cake mixture between the paper cases.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 25 minutes or until risen and just firm to the touch. Leave in the tin for 5 minutes, then transfer to a wire rack to cool.
Beat the cream cheese in a bowl with a wooden spoon until smooth and creamy. Beat in the icing sugar and lemon juice. Spread over the tops of the cakes using a small palette knife and scatter with chopped walnuts to decorate.