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.