1. Preheat oven to 350°F (176°C) and grease three 8-inch cake pans.
2. In a bowl, whisk together flour, baking powder, baking soda, and salt. Set aside.
3. In a large mixing bowl, cream butter and sugar until light and fluffy.
4. Beat in egg whites, vanilla, lime juice, tequila, and lime zest.
5. Alternate adding the flour mixture and milk to the batter until smooth.
6. Divide batter evenly among prepared pans and bake for 25-30 minutes or until a toothpick inserted comes out clean.
7. Cool cakes in pans for 10 minutes, then turn out onto wire racks to cool completely.
8. For frosting, beat butter and shortening until smooth. Gradually blend in powdered sugar, lime zest, and juice.
9. Trim tops of cooled cakes. Layer frosting between each cake layer and coat the outside with remaining frosting.
10. Decorate with lime slices and sugar sprinkles, if desired.