How to Make It
Preheat the oven to 400°. Generously butter and flour a 10-inch springform or fluted cake pan. Toast the almonds on a rimmed baking sheet for about 8 minutes, or until lightly browned. Transfer the almonds to a large plate to cool to room temperature. Reduce the oven temperature to 350°.
In a large mixing bowl, using a fork, blend the flour with the baking powder, lemon zest and a pinch of salt. In a food processor, grind the toasted almonds with the sugar to a fine powder; do not let the nuts turn into a paste. Add the almond powder to the flour mixture in the bowl. Add the carrots to the processor and whir until very finely chopped; add them to the bowl. Add the egg yolks to the bowl and stir until blended.
In a large stainless steel bowl, beat the egg whites with a pinch of salt until they hold firm peaks. Using a rubber spatula, stir one-third of the egg whites into the carrot mixture to loosen it, then gently fold in the remaining whites until the batter is just blended.
Scrape the batter into the prepared pan and bake for 30 minutes, or until the cake is just set. Transfer the cake to a rack and let it cool slightly in the pan. Unmold the cake onto a serving dish. Just before serving, sift confectioners' sugar over the top. Serve warm or at room temperature.