Preheat oven to 350°.
Place 16 paper muffin cup liners in muffin cups. Set aside.
Lightly spoon cake flour into dry measuring cups; level with a knife. Sift together flour and 3/4 cup powdered sugar into a medium bowl; repeat the procedure 2 times.
Beat egg whites and salt with a mixer at high speed until frothy (about 1 minute). Add cream of tartar, and beat until soft peaks form. Add 1/2 cup granulated sugar, 1 tablespoon at a time, beating until stiff peaks form. Sprinkle flour mixture over egg white mixture, 1/4 cup at a time; fold in after each addition. Stir in vanilla and rind.
Divide batter evenly among prepared muffin cups. Bake at 350° for 18 minutes or until lightly browned. Remove from pan; let cool completely on a wire rack.
To prepare frosting, beat butter with a mixer at high speed until fluffy. Gradually add 2 cups powdered sugar; beat at low speed just until blended. Add milk and lemon juice; beat until fluffy. Add more lemon juice as needed to adjust the consistency. Spread 2 tablespoons lemon frosting over each cupcake. Garnish with edible pansies and rosebuds, if desired.