In a large bowl, whisk together the flour, powdered sugar, baking powder, and salt.  Set aside.

In a medium bowl, whisk together the milk, melted butter, vanilla, and egg yolk until well combined. Set aside.

In the bowl of an electric hand mixer, beat the egg whites and cream of tartar on medium-high speed until stiff peaks form, about 3 minutes. Set aside.

Next, pour the liquid mixture into the flour mixture.  Use a silicone spatula to mix just until the flour is moistened.  There will be lumps.  Don’t over mix.

Fold in ⅓ of the whipped egg whites into the flour mixture to loosen the batter, then very gently fold in the remaining egg whites.  Be careful not to overmix and deflate the egg whites.  Don’t overmix the batter.

Place a large nonstick skillet over low heat.  Place four (3-inch) ring molds in the skillet and spray both the skillet and molds well with cooking spray.

Using a half-cup measure, fill each mold about halfway with the batter.

Pour in 1 tablespoon of water around each mold.

Cover the skillet with a lid and cook until the batter rises to the top of the mold, looks set, but is jiggly in the middle and is golden on the bottom – about ten minutes.

Use a spatula to gently release the pancakes from the bottom of the pan.  

Carefully flip the pancakes and molds over, making sure not to spill any batter.  You may need to use tongs to stabilize the mold as you flip.

Cover and cook for another five minutes. 

Use a spatula to remove the pancakes to a serving plate and remove the mold.

Serve immediately with butter and syrup.