This is a simple and delicious, rustic cake that is naturally gluten-free, as it's made with almond meal instead of flour. This also makes the cake protein-rich and nutritious.
Making it does require either a food mill or a food processor, but it has an intense lemon flavor; if you like lemons, you will like this cake. The easiest way to measure out the almonds for this recipe, whether you're using ground almond meal or whole blanched almonds -- is to use three 1/4-pound (100 gram) packages.
Preheat the oven to 360 F (180 C).
Wash the lemons well and gently simmer them in enough water to cover for 1 hour.
They will look a bit "deflated," while the water will be pale yellow and quite bitter. Drain the lemons, and as soon as they are cool enough to handle, trim away the stem ends, and cut them in half crosswise. Scoop out and discard the seeds.
Coarsely chop the lemons (peels and all), and then put them through a food mill, once with a coarse disk and then again with a fine disk. Strain the lemon pulp through a fine-mesh wire strainer by rubbing the lemon paste against the mesh with the back of a wooden spoon or the bottom of a ladle.
If you have ground almonds, skip to the next step. Otherwise, divide the blanched almonds into 3 batches and grind them in a food processor or blender until they form a homogeneous almond meal. Set aside.
Soak a sheet of parchment paper, crumple it, wring it out, and use it to line a 9-inch (22-cm) springform pan. Alternatively, grease the bottom and sides of the pan with butter, then dust lightly with flour, tapping the side of the upside-down pan to remove any excess flour.
Beat the yolks and granulated sugar in a medium bowl until they are pale yellow and frothy, about 3 minutes, and then beat in the lemon cream and the almond meal.
In a separate, clean, dry bowl, whip the whites to firm peaks. Gently fold the whites and baking powder into the lemon mixture and pour the batter into the prepared springform pan.
Bake for about 1 hour, or until a toothpick inserted in the middle comes out a bit sticky but without batter clinging to it -- keep an eye on the cake during the last few minutes, because it can brown suddenly.
When it is done, let cool completely before releasing it from the springform pan. Dust lightly with confectioner's sugar and serve.