Gluten-free carrot cake

Name: Gluten-free carrot cake
Prep time: 20 min
Cook time: 55 min
Total time: 1 h 15 min
Cuisine: British
Ingredients: 140g unsalted butter, softened, plus extra for greasing; 200g caster sugar; 250g carrots, grated; 140g sultanas; 2 eggs, lightly beaten; 200g gluten-free self-raising flour; 1 tsp cinnamon; 1 tsp gluten-free baking powder; 50g mixed nut, chopped; 75g butter, softened; 175g icing sugar; 3 tsp cinnamon, plus extra for dusting