Preheat the oven to 180ºC. Lightly grease and line a 20cm spring form cake tin.
Place the carrots, mascovado sugar, oil, yoghurt and eggs into a food processor and blend until smooth. If you are doing this by hand, grate the carrots into a large bowl and mix well with the sugar, oil, yoghurt and eggs.
Sieve the flour with the baking powder and mixed spice into a mixing bowl and sir in the carrot mixture along with the sultanas. Mix well and pour into the prepared tin.
Bake for 40-50 minutes until a skewer poked into the centre comes out clean. Leave in the tin for 5 minutes before turning out onto a wire rack to cool.
To make the icing, sieve the sugar into a bowl then stir in the yoghurt and orange zest. Drizzle over the cooled cake and serve.