Chocolate Angel Food Cake
["3/4 c. sifted cake flour", "4 generous Tbsp. cocoa", "1 1/4 c. egg whites (room temperature; about 8)", "1/4 tsp. salt", "1 tsp. cream of tartar", "1 1/4 c. sugar, sifted", "1 tsp. vanilla or lemon juice"]
["Preheat oven to 350\u00b0.", "Sift together flour and cocoa; repeat 4 or 5 times until well blended.", "Beat egg whites and salt together; sprinkle with cream of tartar.", "Beat until stiff and holds a peak. Gradually fold in sugar, then flour-cocoa mixture.", "Use an over-under motion with a spatula."]
["cake flour", "generous", "egg whites", "salt", "cream of tartar", "sugar", "vanilla"]