1. Whisk together ricotta, egg yolks, sugar, and lemon zest in large bowl. Set aside.
2. Whisk together flour, potato starch, baking soda, and salt in separate bowl. Sprinkle flour mixture over egg mixture, and fold in with spatula.
3. Beat egg whites and cream of tartar with electric mixer 5 minutes, or until soft peaks form. Fold egg whites into batter.
4. Heat 1 Tbs. butter in large skillet over medium heat. Pour 1/4 cup batter for each pancake into pan. Cook 3 minutes, or until bubbles begin to form and pop on top. Flip, and cook 1 minute more. Repeat with remaining butter and batter.