Allow egg whites to stand at room temperature for 30 minutes. Meanwhile, line twenty to twenty-two 2-1/2-inch muffin cups with paper bake cups. In a medium bowl stir together flour, baking powder, baking soda, and salt. In a 2-cup glass measuring cup combine the buttermilk and the creme de menthe. Set aside.

Preheat oven to 350 degrees F. In a large mixing bowl beat shortening with an electric mixer on medium to high speed for 30 seconds. Gradually add sugar, about 1/4 cup at a time, beating on medium speed until light and fluffy. Beat in vanilla. Add egg whites, one at a time, beating well after each addition. Alternately add flour mixture and buttermilk mixture to shortening mixture, beating on low speed after each addition just until mixture is combined.

Spoon batter into prepared muffin cups, filling each about two-thirds full. Use the back of a spoon to smooth out batter in cups.

Bake for 15 to 18 minutes or until tops spring back when lightly touched. Cool cupcakes in muffin cups on wire racks for 5 minutes. Remove cupcakes from muffin cups. Cool completely on wire racks.

Divide White Chocolate Frosting between two bowls. Tint one portion with green food coloring. Spoon each frosting into a pastry bag fitted with a large star tip. Pipe white and green frostings onto tops of cupcakes to resemble four-leaf clovers. Makes 20 to 22 (2-1/2 inch) cupcakes.

For each 1 cup sour milk, place 1 tablespoon lemon juice or vinegar in a glass measuring cup. Add enough milk to make 1 cup total liquid; stir. Let stand for 5 minutes before using.

If you prefer not to use creme de menthe, substitute a mixture of 1/4 cup milk, 1 teaspoon mint extract, and several drops green food coloring.

Place white baking chocolate in a large mixing bowl; set aside. In a small saucepan heat whipping cream just until simmering. Pour over white baking chocolate; let stand, without stirring, for 5 minutes. Stir until smooth; let stand for 15 minutes. Gradually beat butter into melted white chocolate mixture with an electric mixer on medium to high speed, beating until combined. Gradually beat in powdered sugar until frosting reaches piping or spreading consistency. Makes 3-1/2 cups.