One 15.25-ounce box white cake mix
1 cup all purpose flour
1 cup granulated white sugar
3/4 teaspoon salt
1 1/3 cups water
2 tablespoons vegetable or canola oil
1 teaspoon vanilla extract
1 cup sour cream
4 large egg whites
4½ cups powdered sugar
1½ cups (3 sticks) salted butter, at room temperature
1½ teaspoons vanilla extract (use clear extract, if you really want it white)
3 to 4 tablespoons heavy whipping cream
BAKE THE CAKE:
Preheat oven to 325°F. Grease and flour three 9-inch round pans. It’s easiest to get the cake out of the pans if you line them with rounds of parchment paper too.
In a large bowl, whisk together the cake mix, flour, sugar and salt. Add the remaining ingredients and beat with hand mixer for 2 minutes, or until well blended.
Divide the batter evenly between the prepared pans. Be sure to fill them 3/4 to 1-inch below the top of the pan. Bake 35 to 45 minutes, or until a toothpick inserted into the center comes out clean. Watch closely- check after 35 minutes and then every 5 minutes until they test done.
Time will vary based on which pans you choose to use. Let the cakes cool in the pans for a few minutes, then turn out onto cooling racks to cool completely before frosting.
FROST THE CAKE:
In a large bowl, with a hand or stand mixer, cream together the sugar and butter on low speed. Mix until well blended. Increase the speed to medium and beat for another 3 minutes.
Add the vanilla and 3 tablespoons of cream and continue to beat on medium speed for 1 minute more. Add more cream if needed for desired spreading/piping consistency.
You may find that the layers are a bit moist on the surface, so just be careful when handling and frosting. Place one layer of the cake on a serving plate.
Spoon a dollop of the frosting on the first layer and spread to cover. Place the second layer of cake on the first. Spread another thin layer of frosting on the second layer.
Add the third layer of cake. Spread frosting on the top and then down the sides of the cake to cover.
You should have enough frosting left over to add some decorations. Scoop the remaining frosting into a piping bag and pipe decoratively onto the cake, as desired.
I like to refrigerate the cake until ready to serve (or at least until the day of serving (can make one day ahead).