Instructions

• Preheat the oven to 325°F. Grease and flour three 8-inch round cake pans; line the bottoms with parchment rounds for easiest removal.
• In a large bowl, whisk together the 15.25-ounce box white cake mix, 1 cup all-purpose flour, 1 cup granulated sugar, and 3/4 teaspoon salt until evenly combined.
• Add 1 1/3 cups water, 2 tablespoons vegetable or canola oil, 1 teaspoon vanilla extract, 1 cup sour cream, and 4 large egg whites to the dry ingredients. Beat with a hand mixer (or stand mixer) on medium for 2 minutes, stopping once to scrape down the bowl so the batter is uniform.
• Divide the batter evenly among the three prepared pans. Leave about 3/4 to 1 inch of space between the batter and the top edge of each pan.
• Bake 35 to 45 minutes, checking at 35 minutes and then every 5 minutes, until a toothpick inserted into the center of each cake comes out clean or with a few moist crumbs. Exact time depends on your oven and pans.
• Remove the pans from the oven and let the cakes rest in the pans for about 10 minutes. Run a thin knife around the edges, invert each cake onto a cooling rack, remove the parchment, and let the layers cool completely before frosting.
• For the frosting: in a large bowl with a hand or stand mixer, add 1 1/2 cups (3 sticks) salted butter at room temperature. With the mixer on low, begin adding 4 1/2 cups powdered sugar gradually to prevent a sugar cloud; mix on low until mostly combined.
• After the powdered sugar is incorporated, increase the mixer speed to medium and beat the butter-and-sugar mixture for 3 minutes until light and smooth. Scrape the bowl and paddle as needed.
• Add 1 1/2 teaspoons vanilla extract (use clear vanilla if you prefer a whiter frosting) and 3 tablespoons heavy whipping cream. Beat on medium for 1 minute. If the frosting is too stiff for spreading or piping, add up to 1 more tablespoon of heavy cream (use a little at a time) until you reach the desired consistency.
• When the cake layers are completely cool, place one layer on your serving plate or turntable. Spoon a generous dollop of frosting onto the layer and spread evenly to the edges.
• Place the second cake layer on top of the frosted first layer. Spread a thin layer of frosting over the second layer (this can act as the filling). Add the third layer on top.
• Apply frosting to the top and sides of the stacked cake, smoothing as desired. If you want a neater finish, apply a thin crumb coat first, chill briefly, then finish with a final layer of frosting.
• Reserve some frosting for decoration, transfer it to a piping bag, and pipe decorative details as desired.
• Refrigerate the finished cake until ready to serve. The cake can be made and refrigerated up to one day ahead; cover it when chilling.