1Position a rack in the center of the oven and preheat the oven to 350°F. Lightly butter a 9-inch springform pan.
2Whisk the flour and baking powder into a large bowl. Add the almond flour and whisk again. Beat the egg yolks and granulated sugar in a separate large bowl with an electric hand mixer set at high speed, scraping down the sides as needed, until the mixture is thickened and pale yellow, about 2 minutes. (A hand mixer or a whisk works best. In a standing mixer, the yolks may not have enough volume for the whisk attachment to reach them in the bottom of a standing mixer’s bowl.)
3Using a sturdy wooden spoon, gradually stir in the almond flour mixture, followed by the shredded carrots. If using the vanilla extract, stir in now. The batter will be quite thick at this point.
4Using clean beaters, whip the egg whites with the salt in a medium grease-free bowl with the hand mixer set on high speed until they form stiff , but not dry, peaks. Using a rubber spatula, stir about one-fourth of the whites into the carrot mixture to lighten it. Fold in the remaining whites. Spread the batter evenly in the prepared pan.
5Bake until the cake is golden brown and has shrunk slightly from the pan, about 1 hour. Let the cake cool on a wire rack for 30 minutes. Remove the sides of the pan and let cool completely. The cake can be wrapped in plastic wrap and stored at room temperature for up to 2 days. If possible, let the cake stand for at least 12 hours before serving.
6Just before serving, sift confectioners’ sugar over the cake. Slice and serve with a dollop of the mascarpone cream, topped with a grating of orange zest, if desired.
1To make the mascarpone cream: Work the mascarpone and confectioners’ sugar together in a small bowl until combined. Let stand at room temperature for about 1 hour to remove its chill.