Preheat oven to 350°F (176°C) and prepare a cupcake pan with cupcake liners.
In a large mixing bowl, cream butter and sugar together until light in color and fluffy, about 3-4 minutes.
Add sour cream, vanilla extract and orange zest and mix until well combined.
Add egg whites in two batches, mixing until well combined after each. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated.
Combine dry ingredients in a separate bowl, then combine the orange juice, tequila and milk in a small measuring cup.
Add half of the dry ingredients to the batter and mix until well combined. Add the orange juice mixture and mix until well combined. Add remaining dry ingredients and mix until well combined. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated.
Fill the cupcake liners about halfway. Bake for 15-17 minutes, or until a toothpick inserted comes out with a few crumbs.
Make the Frosting:
Beat the butter and shortening together until combined and smooth.
Add about half of the powdered sugar and mix until smooth.
Add the tequila and mix until smooth.
Add the remaining powdered sugar and mix until smooth.
Divide the frosting in half. Add the orange zest and juice to one half and the grenadine to the other. Add some orange icing color to the orange frosting, if desired.
Put each of the two frostings into two piping bags, then place both piping bags into one larger bag fitted with a large closed star tip (I used Ateco 844). Pipe swirls of frosting on top of each cupcake, then top them with a cherry and quarter slice of orange.
If you prefer an all butter frosting, simply substitute the ½ cup of shortening with ½ cup of butter (totaling 1 ¼ cups of butter for the frosting).
Store cupcakes in the refrigerator. Serve at room temperature.