Preheat oven to 350°F. Start beating egg whites in a stand mixer on high. Add salt and cream of tartar. Beat until there are stiff peaks. You should be able to turn it upside down and it will stay. It will take around 5-10 minutes.
While the egg whites are beating. With a hand beater, beat the egg yolks and sugar until pale yellow. Add lemon zest and vanilla.
Sift the almond flour and baking powder. Mix half of the almond flour/baking powder into the egg yolks. Add some of the egg whites. Fold in the rest of the egg whites and almond flour. Don’t overmix or you’ll let too much air out of the egg whites.
Separate into two greased spring form pans and bake 30-40 minutes. It should be dry if a toothpick is inserted in the center.