Whisk flour, fennel seeds, baking powder, sugar, and salt in a small bowl.
Combine ricotta, milk, egg yolks, and vanilla in a separate mixing bowl.
Add the dry ingredients to the ricotta and milk mixture, stirring gently until just combined.
Beat the egg whites with a handheld electric mixer until stiff.
Stir a tbsp of the egg whites into the pancake batter to lighten the batter, then fold in the remaining whites with a spatula.
Heat a griddle over medium-high heat. Melt a small bit of butter in the pan, just enough to coat the surface. Use a 1/4-cup measure to pour batter onto the hot griddle.
Spoon a tbsp of the strawberry preserve in the center and pour a small amount of batter just to cover it.
Cook the pancakes for about 3 or 4 minutes, until the undersides are golden and you see a few bubbles popping through the pancakes. Flip the pancakes and cook another 2 to 3 minutes, until golden. Repeat with the remaining pancakes.
Serve them immediately with powdered sugar, maple syrup and some fresh strawberries.