In a bowl whisk together egg yolks, ricotta, sugar and zest.
Add flour, and stir the mixture until it is just combined.
In a bowl with an electric mixer, beat the egg whites with a pinch of salt until they hold stiff peaks. Whisk about ¼ of them into ricotta mixture, and fold in the remaining whites gently but thoroughly.
Heat the griddle over med-low heat, working in batches, pour the batter onto the griddle by ¼ cup measures and cook the pancakes for 1 minute on each side, or until they are golden, brushing the griddle with some of the melted butter as necessary.
Transfer pancakes as they are cooked to a heat-proof platter and keep them warm.
Serve the pancakes with a dusting of sugar, dollop of whipped cream and warm maple syrup.