Carrot Cake with Lime Mascarpone Icing

Serves 8 to 10
1 1/4 cups unsalted butter, room temperature, plus more for pan
2 cups light-brown sugar
5 large eggs, separated
Zest and juice of 1 orange
1 slightly heaped teaspoon baking powder
1 heaped teaspoon ground cinnamon
Pinch of ground cloves
Pinch of ground nutmeg
1/2 teaspoon ground ginger
1 cup ground almonds
4 ounces shelled walnuts, chopped, plus more for garnish
1 1/2 cups self-rising flour, sifted
10 ounces carrots, peeled and coarsely grated
Sea salt
4 ounces mascarpone cheese
8 ounces cream cheese, room temperature
1 scant cup confectioners' sugar, sifted
Zest and juice of two limes
Directions
Preheat oven to 350 degrees. Butter a 9-by-2-inch square baking pan and line with parchment paper. Set aside.
In the bowl of a food processor, pulse together butter and brown sugar until pale and fluffy. Add egg yolks, one at a time, pulsing after each addition. Add orange zest and juice; pulse to combine. Add baking powder; pulse to combine. Add cinnamon, cloves, nutmeg, and ginger; pulse until mixture is well combined.
In a large bowl whisk egg whites with a pinch of salt until stiff. Gently fold in almonds, walnuts, flour, and carrots; fold cake mixture into egg white mixture. Pour batter into prepared pan and bake until golden and a toothpick inserted into the center comes out clean, about 1 hour. Let cake cool in the pan 10 minutes before turning out onto a wire rack to cool completely.
In a medium bowl, mix together mascarpone, cream cheese, confectioners' sugar, and lime zest and juice. Spread mixture over top of cooled cake and sprinkle with chopped walnuts.