Allow butter and eggs to stand at room temperature for 30 minutes. Meanwhile, line eighteen 2-1/2-inch muffin cups with paper bake cups. In a medium bowl stir together flour, baking powder, baking soda, and salt. Set aside.
Preheat oven to 350 degrees F. In a large mixing bowl beat butter with an electric mixer on medium to high speed for 30 seconds. Add sugar, lemon peel, lemon juice, and liqueur; beat until combined, scraping sides of bowl occasionally. Add eggs, one at a time, beating well after each addition. Alternately add flour mixture and buttermilk, beating on low speed after each addition just until combined. Tint batter pink with a few drops of food coloring.
Spoon batter into prepared muffin cups, filling each about three-fourths full. Use the back of a spoon to smooth out batter in cups.
Bake for 15 to 18 minutes or until a wooden toothpick inserted in centers comes out clean. Cool cupcakes in muffin cups on wire racks for 5 minutes. Remove cupcakes from muffin cups. Cool completely on wire racks.
To make 2/3 cup sour milk, place 2 teaspoons lemon juice or vinegar in a glass measuring cup. Add enough milk to make 2/3 cup liquid; stir. Let the mixture stand for 5 minutes before using.
Allow butter to stand at room temperature for 30 minutes. In a large mixing bowl beat butter with an electric mixer on medium speed until smooth. Gradually add 2 cups of the powdered sugar, beating well. Beat in milk, liqueur, and food coloring. Gradually beat in the remaining powdered sugar until frosting reaches spreading or piping consistency.