Cake flour is the best for this recipe. If you use all purpose flour, the madeleines will not be as tender. If you don't have cake flour on hand, measure out one cup of all purpose flour and take out 2 Tablespoons of the flour. Add 2 Tablespoons of cornstarch to the flour and sift the flour and cornstarch together about 3 times. This is a great substitute for cake flour.
For a different flavor, add orange or lemon zest to the batter. You can also add almond extract, rum, bourbon, cognac, etc in addition or in place of the vanilla extract. You can make the madeleines chocolate by substituting ¼ cup of flour for unsweetened cocoa powder.