Beat the ricotta cheese, egg yolks, butter, vanilla and lemon extract in a medium bowl until well mixed. Combine the flour, sugar, lemon zest and salt in a small bowl and mix well. Stir the flour mixture into the ricotta mixture, mixing carefully.
Whip the egg whites in a clean, dry medium bowl until stiff peaks form. Mix a small amount of the beaten egg whites into the batter with a rubber spatula. Fold the remaining egg whites in carefully, using as few strokes as possible.
Spoon 1/4 cup batter for each pancake onto a hot, lightly greased griddle or skillet. Sprinkle a few frozen blueberries on the surface of each pancake.
Cook until golden brown. Flip the pancakes when small bubbles appear on the top and bottoms are firm. Cook until golden brown.
Variation: Omit the blueberries and increase the lemon zest by 1 teaspoon for lipsmaking lemony pancakes. Serve with raspberry maple syrup.