Heat your oven to 375ºF. Grease and flour twenty-four 3" madeleine molds. If you have a pan with just 12 molds, just make two batches.
Beat the egg white in small bowl with an electric mixer on medium speed until it is foamy. Beat in 1/4 cup of the granulated sugar, 1 tablespoon at a time. Continue beating until the mixture is very stiff and glossy. Set it aside.
Except for the powdered sugar, which you will use once the madeleines are baked, beat the remaining ingredients, including the remaining 1/4 cup of granulated sugar, with the egg yolk in a medium bowl on high speed for 2 minutes, scraping the bowl occasionally
Fold in the egg white mixture.
Ideally, you should chill the batter for about 30 minutes before you bake them. It really does make the end result fluffier.
Fill the molds about two-thirds full. The batter doesn't have to reach the end of the molds. It will travel as it bakes. Tap the filled madeleine pan firmly on the counter to remove any air bubbles in the batter.
Bake them for 10 to 12 minutes or until the edges of the cakes are light brown. Cool the madeleines for 1 to 2 minutes, then remove them from the molds and place them on a wire rack to cool.
Cool the madeleines completely and sprinkle them with the powdered sugar just before serving.