• Preheat your oven to 350°F (or 180°C).

  Line a 9×13 pan with parchment and spray with nonstick spray that contains flour.
• In the bowl of your stand mixer, add all of the homemade white cake mix, whole milk, oil and vanilla extract. Mix on medium-low speed until the ingredients are incorporated. Add in the egg whites and mix until combined. Scrape down the sides and bottom of the bowl and mix one last time.
• Pour the cake batter into the prepared pan and spread evenly with a spatula. Bake on the middle rack of your preheated oven for 28 to 30 minutes or until a tester comes back clean (maybe with a few crumbs attached) after inserted into the middle of the cake.
• Once baked, remove and let cool for about 30 to 45 minutes before using the parchment paper to lift the cake out of the pan and transferring it to a wire rack to finish cooling.

MAKE THE VANILLA BUTTERCREAM FROSTING:

• In the bowl of your stand mixer, fitted with the paddle attachment, add the softened butter. With your mixer on medium-speed, cream the butter until smooth and light in color. About 2 minutes.
• Gradually add in the sifted powdered sugar, one large spoonful at a time. Mixing on low speed after each addition. Next add in a pinch of fine salt and the vanilla extract. Continue to mix on low speed to combine. Lastly add a little milk at a time until you’ve reached your desired consistency.

  Note: If you need a stiffer frosting, simply add more sifted powdered sugar. Likewise, if your frosting is too stiff, continue adding a little more milk.

BUILD THE CAKE:

• Add about 2/3 of the buttercream to your cooled cake- transfer the remaining frosting to a pastry bag fitted with a large star tip. Use an offset spatula to spread the buttercream evenly over the cake.
• Next arrange the blueberries (I toss them in a little powdered sugar first – this is totally optional) and arrange in the top left corner of the frosted cake for the “stars”. Then arrange the raspberries in rows as the “stripes”. Lastly, pipe stars in between the rows of raspberries.