Preheat your oven to 325 degrees F. Spray three 8-inch round cake pans with non-stick spray, line the bottom with parchment paper and spray again. Set aside.
In a medium sized mixing bowl, combine the flour, baking powder and salt. Whisk to combine. Set aside.
In a stand mixer fitted with the paddle attachment, combine the granulated sugar, the mint lime sugar and the butter. Beat on medium-high speed for 2 to 3 minutes, scraping down the sides and the bottom of the bowl half way through. You want the sugar and butter mixture to get light in color and texture.
Add the sour cream and mix until well blended.
With the mixer on low speed, alternately add the dry ingredients and the buttermilk, starting and ending with the dry ingredients. The mixture will be thick.
In a stainless steel bowl, whisk the egg whites until stiff peaks form.
Fold the egg whites into the cake batter.
Evenly distribute the cake batter into the prepared pans, about 16 ounces in each of the 8-inch pans. (You can also do four 6-inch pans with about 12 ounces of batter in each pan).
Bake the cake layers for about 23 to 25 minutes, or until a toothpick inserted in the center of the cake comes out with a few moist crumbs on it.
Allow the cake to cool in the pans on a wire rack for about 10 to 15 minutes before inverting the layers onto the racks to cool completely. To ensure your layers don't stick to your cooling racks, spray the racks with nonstick spray. Allow the cakes to cool completely before using or wrapping plastic wrap to store in the freezer for another day.