1½ cups ground almonds (you want them on the verge of fine)
1½ cups granulated sugar (use only a cup if using oranges instead of grapefruits)
Flour (or matzo meal or cornstarch)
Wash the grapefruits, then bring them to a boil in a pot big enough to keep the fruits submerged in water. Simmer until the skin is shiny and almost see-through, turning the fruit every 3o minutes or so. It will take about two hours total.
Drain and let the cooked fruit cool for at least 3o minutes. They are hotter inside than you might anticipate.
Preheat the oven to 400° Rub the inside and sides of a cake pan with butter, then coat the buttered pan with flour. (Ideally, use a cake pan with a removable base.)
Dig through the flesh and remove the seeds. Puree both the flesh and rind with a blender or immersion blender. Or push the flesh and rind through a strainer if you don’t have that kind of machinery.
Beat the eggs together in a large bowl. Add the almonds, sugar, baking powder and pureed fruit and mix well. Bake until the middle of the cake is almost set and no longer wet, about an hour.
Let cool and serve with sweetened whipped cream. Or yogurt. Something rich and sweet. The cake keeps in the fridge for at least three days.