To make raspberry syrup, thaw berries, but don’t drain. Place the berries in a blender and process until smooth. Press berries through a fine-mesh strainer and discard the seeds. Cook and stir juice over medium heat until heated through. Stir in the maple syrup and set aside.

To make pancakes, stir together flour, baking powder, lemon zest and salt.

In a separate bowl, slightly beat the egg yolk. Stir in melted butter and milk. Add this egg yolk mixture to the flour mixture and stir just until moistened (batter will be lumpy).

In another mixing bowl, beat egg whites with an electric mixer on medium speed until stiff peaks form. Gently fold the egg whites into the flour mixture just until barely combined - do not overmix.