Best of the best carrot cake

Method

1. Heat the oven to 180°C fan/gas 6. Finely zest the orange into a small bowl. Put the sultanas in another small bowl and squeeze the juice of the orange over them. Put the walnuts and pecans in a small baking dish and toast in the oven for 10 minutes, turning halfway through. Leave to cool, then finely chop.
2. Put the butter for the frosting in a pan over a medium heat and cook until sizzling, golden and toasty. This should take around 8 minutes; the white milk solids should become amber and sink to the bottom. Keep an eye on it; if it burns it will taste bitter and you’ll have to start again. Leave to cool and firm up completely (see Make Ahead).
3. Heat the oven to 160ºC fan/gas 4 and line both cake tins with baking paper. Using a stand mixer or electric hand mixer, whisk together the oil, eggs, muscovado sugar and vanilla for 7 minutes. Meanwhile, finely grate the carrots (no need to peel), then gently stir them into the whisked egg mixture along with the soaked sultanas and any leftover orange juice. Sift the dry ingredients and spices into the bowl and gently stir to combine, then stir in the soured cream, reserved orange zest and chopped toasted nuts.
4. Divide the mixture equally between the cake tins and bake for 40-45 minutes or until a skewer pushed into the centre comes out clean. Carefully remove the cakes from the tins and leave to cool completely on a wire rack.
5. While the cakes cool, make the frosting. Put the cooled brown butter in a stand mixer with a beater attachment and whip on a low-medium speed. Sift in the icing sugar, a third at a time, until completely combined, then beat in the mascarpone, followed by the soured cream and grated nutmeg.
6. Once the cakes have completely cooled, use a serrated knife to slice off the raised dome of one of the cakes to get a flat surface. Spread half the frosting neatly over the newly flattened surface, then carefully place the second cake on top and add the rest of the frosting to the top. To finish, finely zest the half grapefruit, lime and orange over the icing to add a wonderfully fresh and colourful styling to the cake.