1. Preheat the oven to 350°F (176°C). Line three 8-inch cake pans with parchment paper in the bottom and grease the sides.
2. Combine the flour, baking soda, baking powder and salt in a medium sized bowl. Set aside.
3. Cream the butter and sugar in a large bowl on medium speed about 2-3 minutes, until light and fluffy.
4. Add the egg whites and vanilla and beat on medium speed until well combined. Scrape down the sides of the bowl, as needed, to make sure everything is combined.
5. Add about one-third of the flour mixture to the batter and beat on medium speed until incorporated.
6. Add the milk and beat on medium speed until incorporated.
7. Add another third of the flour mixture to the batter and beat on medium speed until incorporated.
8. Add the lime juice, tequila and lime zest and beat on medium speed until incorporated.
9. Add the remaining dry ingredients and beat on medium speed until well combined and smooth.
10. Divide the batter evenly between the cake pans and spread evenly.
11. Bake the cakes for 25-30 minutes, or until a toothpick inserted into the middle of the cakes comes out clean. Don’t be alarmed if the tops are a little browned, that is normal.
10. Remove the cakes from the oven and allow to cool for 1-2 minutes, then remove to a cooling rack to cool completely.
11. To make the frosting, add the butter and shortening to a large mixer bowl and beat until smooth.
12. Slowly add about half of the powdered sugar, mixing until well combined and smooth.
13. Add the lime zest and lime juice and mix until well combined and smooth.
14. Slowly add the remaining powdered sugar and mix until well combined and smooth.
15. Add additional lime juice, if needed, to thin out the frosting.
16. To put the cake together, trim the tops of the cakes so they are flat. The cake layers should be pretty flat, but if there’s a dome, trim it off.
17. Place the first layer of cake on a serving plater, then top it with about 1 cup of frosting and spread into an even layer.
18. Add the second layer of cake and another layer of frosting.
19. Top the cake with the remaining cake layer and frost the outside of the cake. I used my 9 inch offset spatula to create the lines on the side of the cake, but feel free to leave that off.
20. Finish off the cake with some swirls of the remaining frosting and lime slices. I also sprinkled some coarse sugar sprinkles around the top of the cake to mimic the look of salt, but that’s optional.