How to Make It
Let egg whites come to cool room temperature. Preheat oven to 375°. Spread rice flour in a pie pan and bake, stirring a few times, until light golden, 20 to 25 minutes. Let flour cool (chill to speed up), then stir in 1/4 cup superfine sugar, lemon zest, and vanilla seeds. Sift mixture or rub through a coarse strainer to release oil from zest (add any flavorings that don't go through back to flour); set aside.
Butter a 10-in. tube pan with a removable bottom and dust with 1 1/2 tbsp. superfine sugar; set aside. In a small bowl, blend remaining 1 1/2 cups superfine sugar with the cream of tartar.
In bowl of a stand mixer fitted with the whisk attachment, beat egg whites with salt on medium speed. When frothy, gradually add sugar mixture over about 1 1/2 minutes. Add vinegar in a slow stream. Continue to beat on medium speed until whites are glossy and hold very soft peaks that fold over when whisk is lifted, 8 to 9 minutes.
With a large plastic spatula, transfer whites to a wide bowl. Sprinkle with one-third of flour mixture and gently fold in. Repeat with remaining flour. Pour batter into pan; spread level.
Bake cake 15 minutes. Reduce heat to 350° and continue to bake until cake is deep golden, begins to pull from sides of pan, and a toothpick inserted in center comes out clean, 20 minutes. Set on a cooling rack until barely cool enough to handle, 10 minutes. Place a cake stand over pan, gently invert cake, and remove pan bottom. Let cool completely, 1 hour.
Sift powdered sugar over cake and serve with candied kumquats.
*Find at supermarkets and bobsredmill.com.
Make ahead: Through step 5, 1 day.
Nutritional analysis is per serving with 2 tbsp. kumquats & 1 tbsp. syrup.