Beat the ricotta with the milk and egg yolks in a large bowl. Stir in the flour, sugar, baking powder, lemon rind and blueberries until well combined.
Whisk the egg whites with a hand-held electric whisk in a separate large, grease-free bowl until they form soft peaks, then gently fold into the ricotta mixture with a large metal spoon.
Heat a little butter in a heavy-based frying pan over a medium heat. Add about one-quarter of the batter to the pan to make 3–4 pancakes about 7 cm (3 inches) in diameter and cook for 1–2 minutes on each side until golden and cooked through. Transfer the pancakes to a baking sheet and keep warm in a low oven while you repeat with the remaining batter, adding a little more butter to the pan as necessary.
Serve 3–4 pancakes per person, with lemon juice or maple syrup.