Beat egg whites at medium-high speed of a mixer until foamy. Gradually add 1/4 cup sugar, 1 tablespoon at a time, beating until stiff peaks form. Set aside.
Beat 1/2 cup sugar and margarine at medium speed of a mixer until well-blended (about 5 minutes). Add flour, rind, juice, rosemary, and salt, and beat well. Add egg yolks and milk, and beat well. Gently stir one-fourth of egg white mixture into batter; gently fold in remaining egg white mixture. Spoon into 6 (6-ounce) custard cups coated with cooking spray. Place cups in a baking pan, and add hot water to pan to a depth of 1 inch. Bake cakes at 350° for 45 minutes or until set. Remove cups from pan, and sprinkle with powdered sugar.