Step 1: Preheat the Oven and Prepare the Pans

• Preheat the oven: Preheat your oven to 350°F (175°C).
• Prepare the pans: Grease and flour two 9-inch round cake pans or line them with parchment paper for easy removal.

Step 2: Mix the Dry Ingredients

• Combine the dry ingredients: In a medium bowl, whisk together the all-purpose flour, baking powder, and salt. Set aside.

Step 3: Cream the Butter and Sugar

• Cream the butter and sugar: In a large mixing bowl, use a hand mixer or stand mixer to cream the softened butter and granulated sugar together until light and fluffy, about 3-4 minutes.
• Add the egg whites: Gradually beat in the egg whites, one at a time, ensuring each is fully incorporated before adding the next.

Step 4: Add Vanilla and Sour Cream

• Mix in vanilla and sour cream: Stir in the vanilla extract and sour cream until the mixture is smooth and well combined.

Step 5: Combine Wet and Dry Ingredients

• Alternate adding dry ingredients and milk: Slowly add the dry ingredients to the wet mixture in three batches, alternating with the milk. Start and end with the dry ingredients, mixing just until combined. Be careful not to overmix.

Step 6: Bake the White Cake

• Transfer the batter to the pans: Evenly divide the batter between the prepared cake pans and smooth the tops with a spatula.
• Bake: Place the pans in the preheated oven and bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean or with just a few moist crumbs.
• Cool: Remove the cakes from the oven and allow them to cool in the pans for 10 minutes. Then, transfer the cakes to a wire rack to cool completely.

Step 7: Make the Vanilla Buttercream Frosting (Optional)

• Beat the butter: In a large mixing bowl, beat the softened butter with a hand mixer or stand mixer until creamy and smooth.
• Add powdered sugar: Gradually add the powdered sugar, 1 cup at a time, while mixing on low speed.
• Add vanilla and milk: Stir in the vanilla extract and 2 tablespoons of milk or heavy cream. Increase the mixer speed and beat for an additional 2-3 minutes until light and fluffy. If the frosting is too thick, add more milk, 1 tablespoon at a time, until the desired consistency is reached.

Step 8: Assemble the Cake

• Frost the cake: Once the cakes are completely cooled, level the tops with a serrated knife if necessary. Place one cake layer on a serving plate, spread a generous amount of frosting on top, and place the second cake layer on top. Frost the top and sides of the cake evenly.
• Decorate (optional): You can decorate the cake with sprinkles, fresh fruit, or edible flowers for a special touch.