Carrot cake with lime mascarpone icing

This carrot cake is an exceedingly good cake made all the more pleasing by the twist of lime mascarpone icing. It's delicious, it works and it's better than any other carrot cake I've tried. I would normally bake this in a square or round cake tin, but for the picture I used a lovely old loaf tin and it came out looking gorgeous.

Serves 8-10
285g / 10oz unsalted butter, softened
285g / 10oz light brown soft sugar
5 large free-range or organic eggs, separated
Zest and juice of 1 orange
170g / 6oz self-raising flour, sifted
1 slightly heaped teaspoon baking powder
115g / 4oz ground almonds
115g / 4oz shelled walnuts, chopped, plus a handful for serving
1 heaped teaspoon ground cinnamon
A pinch of ground cloves
A pinch of ground nutmeg
0.5 teaspoon ground ginger
285g / 10oz carrots, peeled and coarsely grated
Sea salt

For the lime mascarpone icing
115g / 4oz mascarpone cheese
225g / 8oz full-fat cream cheese
85g / 3oz icing sugar, sifted
Zest and juice of 2 limes

Preheat the oven to 180°C/350°F/. Grease and line a 22cm/9-inch square cake tin or a round equivalent with greaseproof paper. Beat the butter and sugar together by hand or in a food processor until pale and fluffy. Beat in the egg yolks one by one, and add the orange zest and juice. Stir in the sifted flour and baking powder, and add the ground almonds, walnuts, spices and grated carrot and mix together well.

In a separate bowl, whisk the egg whites with a pinch of salt until stiff, then gently fold them into the cake mix. Scoop the mixture into the prepared cake tin and cook in the preheated oven for about 50 minutes until golden and risen. You can check to see if the cake is cooked by poking a cocktail stick into it. Remove it after 5 seconds and if it comes out clean the cake is cooked; if slightly sticky it needs a bit longer, so put it back in the oven. Leave the cake to cool in the tin for 10 minutes, then turn it out on to a rack and leave for at least an hour.

Mix all the icing ingredients together and spread generously over the top of the cake. Finish off with a sprinkling of chopped walnuts.