Preheat oven to 375 degrees. In the bowl of a mixer fitted with the whisk attachment, whisk egg yolks on high speed with 1/2 cup sugar until thick and pale, about 7 minutes; set aside. In a clean bowl, whisk egg whites on medium speed until foamy; slowly add remaining sugar and cream of tartar. Whisk on high speed until stiff peaks form, 10 minutes.Advertisement
Transfer egg whites to a large bowl. Gently fold in yolk mixture, and then fold in cake flour in 3 additions. Using a piping bag fitted with a 1/2-inch-wide pastry tip (such as Ateco #6), pipe batter onto a parchment-lined baking sheet, about 4 inches long for each ladyfinger, spacing them 1 inch apart from the center of each "finger." Bake 10 minutes or until lightly golden. Allow to cool completely on a wire rack.