Remove the zest from half of the lemon (keep the lemon for other use). Separate the eggs. Mix together the flour, sugar, salt, lemon zest, ricotta, and egg yolks in a large bowl until thoroughly combined. Beat the egg whites in a separate bowl until stiff, then fold the egg whites into the ricotta mixture.
Heat a non-stick griddle to medium-high heat (you can use a flat-bottomed frying pan if you don't have a griddle). Spread a knob of butter on the hot griddle then ladle about 4 fl oz (120ml or ½ cup) of the pancake mixture onto the griddle and cook for about 2 minutes per side, until golden. Keep the pancakes warm in a low oven and repeat until the pancake mixture is used up. Serve warm with butter or Maple syrup.