In a medium bowl, sift together the flour, baking powder, baking soda and salt. Set aside.
In the bowl of your mixer, beat the butter until creamy. Add the sugar and beat again for about 3-5 minutes on medium high speed until it turns white and fluffy. Add in the egg whites one at a time and mix on medium until each one is fully mixed in.
Add the extracts, milk, sour cream and vinegar to the butter mixture and mix on medium speed until combined. Add the dry ingredients and mix until jut combined.
Split the batter evenly into three bowls. Tint one bowl red and one bowl blue. To get the tie-dyed effect you will layer scoops of colored batter into the cake pans. One scoop red followed by one scoop white and one scoop blue, repeated until each pan has about 1/3 of each color of batter.
Bake for about 35-40 minutes, until the top of the cake springs back when lightly touched and a toothpick comes out clean from the center.
Allow to cool in the pan on a cooling rack for about 10 minutes before turning upside down on the cooling rack to cool the rest of the way. Cooling the cakes upside down levels out the top so you have nice flay layers that make getting a level cake very easy.
Once the cake has cooled, you can either wrap the layers in plastic wrap and store in the fridge overnight (chilled layers are typically easier to frost) or you can frost them the same day.
On a cake stand or plate, place one layer top down. Add generous dollop of frosting on top and smooth evenly. Add the second layer and another dollop of frosting smoothed over the top. Add the top layer upside down and another dollop of frosting on top. Smooth a thin layer of frosting around the outside of the entire cake as a crumb coat. Chill in the fridge for 15-20 minutes.
Frost the outside of the cake. If you have a bench scraper or cake spatula, you can use this to get nice smooth sides and sharp corners at the top of the cake.
Use a large star tip, such as the Wilton 1M, to pipe a border around the top. Press sprinkles around the bottom of the cake and, if desired, sprinkle around the top border.
How to Make Frosting
In the bowl of a stand mixer, beat the butter until light and fluffy. Scrape down the sides of the bowl and add the powdered sugar 1 cup at a time, mixing on low speed until incorporated. Once all the sugar has been added, add the salt and the vanilla. Increase the speed to medium and beat for 2-3 minutes.
If desired, tint a small amount of frosting red and a small amount blue for the border at the top of the cake.