For raspberry syrup, thaw the 2 cups berries but do not drain. Place the berries in a food processor or blender. Cover and process or blend until berries are smooth. Press berries through a fine-mesh sieve into a small saucepan. Discard seeds. Cook and stir over medium heat just until heated through. Stir in maple syrup; set aside.
To make pancakes, in a medium bowl stir together flour, baking powder, lemon peel, and salt. Make a well in the center of flour mixture; set aside. In a small bowl combine egg yolk, milk, and the 1/4 cup melted butter. Add egg yolk mixture all at once to flour mixture. Stir just until moistened (batter should be slightly lumpy).
In a medium mixing bowl beat egg whites with an electric mixer on medium speed until stiff peaks form (tips stand straight). Gently fold egg whites into batter, leaving a few fluffs of egg white. Do not overmix.
For standard-size pancakes, pour 1/4 cup of the batter onto a hot lightly greased griddle or large heavy skillet. (Or for mini pancakes, pour about 1 tablespoon batter onto hot lightly greased griddle or large heavy skillet.) Cook over medium heat about 2 minutes on each side or until pancakes are golden brown, turning to second sides when pancakes have bubbly surfaces and edges are slightly dry. Serve with raspberry syrup and, if desired, butter and fresh raspberries.