The Bojon Gourmet

0 Position a rack in the center of the oven and preheat to 350º. Butter an 8" round cake pan, and line the bottom with a round of parchment paper.
1 In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter, sugar and zest until light and fluffy, 5 minutes on medium, scraping down the sides of the bowl occasionally. Add the yolks one at a time, mixing to combine.
2 Meanwhile, in a food processor or coffee grinder, grind the almonds with the tapioca as finely as possible without turning them into almond butter. Add the baking powder and salt to combine.
3 On low, stir the almond mixture into the butter mixture. Gradually add the lemon juice and vanilla extract. Scrape down the paddle and sides of the bowl.
4 In a clean, large bowl, whip the egg whites and cream of tartar until soft peaks form. With a large rubber spatula, stir a third of the egg whites into the batter, then fold in the rest just to combine.
5 Pour the batter into the prepared pan. Bake until the top springs back when pressed with a finger, about 45 minutes. Let cool ten minutes. Invert a plate over the top of the cake, and, using oven mitts, flip the whole thing over. Remove the cake pan and the paper, and let the cake cool completely, about 1 hour, before serving.