Heat oven to 180°C (170°C for dark or non-stick pans). Place paper baking cup in each of 24 regular-size muffin cups.
Make and bake cake mix as directed on box for cupcakes, using water, oil and egg whites and adding lemon peel and lemon juice.Cool 10 minutes; remove cupcakes from pans to cooling racks.Cool completely.
In small microwavable bowl, microwave white chocolate uncovered on High about 1 minute, stirring once, until softened and chips can be stirred smooth; cool. In large bowl, beat butter and powdered sugar with electric mixer on low speed. Gradually add melted chips, beating on high speed until frosting is smooth and spreadable. Frost cupcakes. Garnish with flowers.