How to Make It
Preheat oven to 325°.
Combine baking powder, salt, 7 ounces sugar, and flour in a large bowl, stirring with a whisk until mixture is well combined.
Combine orange rind and next 5 ingredients (through egg yolks) in a medium bowl, stirring with a whisk. Add rind mixture to flour mixture, stirring until smooth.
Place egg whites in a large bowl; beat with a mixer at high speed until foamy. Add cream of tartar; beat until soft peaks form. Gradually add remaining 1 ounce sugar, beating until stiff peaks form. Gently stir one-fourth of egg white mixture into flour mixture; gently fold in remaining egg white mixture.
Spoon batter into an ungreased 10-inch tube pan, spreading evenly. Break air pockets by cutting through batter with a knife. Bake at 325° for 45 minutes or until cake springs back when lightly touched. Invert pan; cool completely. Loosen cake from sides of pan using a narrow metal spatula. Invert cake onto plate. Sift powdered sugar over top of cake.