Preheat oven to 350F. Butter and flour three 6 inch round pans. Whisk together the flour, baking powder, baking soda, and salt. Set aside.
Mix vinegar, sour cream, and milk in a small bowl and set aside.
Cream butter and sugar in a stand mixer using a paddle attachment.
Add extract and egg whites one at a time mixing until incorporated before adding the next. Scrape the bowl down.
Add flour and milk mixtures to the butter in alternating batches.
Bake at 350F for about 30 minutes or until the centers are set and springy.
For the Frosting:
Whisk together egg whites and sugar in a glass bowl. Set over a pot of simmering water making sure the water doesn't touch the bowl.
Whisk over hear until a thermometer reads 165F. Transfer mixture to a stand mixer fitted with a whisk attachment and mix on high for a few minutes then stitch to medium low until room temp.
Switch to a paddle attachment and add butter a tablespoon at a time. Scrape bowl down and remember to beat a few seconds just before transferring to piping bag.
For the Assembly:
Add buttercream to the first layer. Place second layer on top and repeat the process until cake is three layers tall.
Cover cake in buttercream and smooth.
Use a large star tip to pipe dollops on top
Add confetti sprinkles to the bottom third of the cake's side and sprinkle on dollops.
If you’re not using 6-inch pans, double the recipe for 8-inch pans or triple the recipe for 9-inch pans.
You can substitute whole milk yogurt for the sour cream if desired, I do it all the time and can’t really tell the difference.
To get FLAT layers that are moist inside and out try using cake strips! You can buy a set on theshop page or make your own from foil and paper towels at home. I made a wholeblog post on it so check it out if you’re interested!
If you don't want to make Italian buttercream you can use the American variety. I just prefer the smoothness and taste of the Italian variety. I know it's a pain to make two different types of frosting though!