Heat oven to 300 degrees F. (this is not a typo, I baked these at a lower temp to avoid the dome on the cakes.)
Generously butter 3 x 7 inch round cake pans. Sprinkle with flour, and tap out the excess; set aside. In a medium bowl, whisk cake flour, salt, and cocoa; set aside.
In the bowl of an electric mixer fitted with the paddle attachment, combine the sugar and oil, and beat on medium speed until well combined.
Add eggs, one at a time, beating well after each addition.
Add food coloring and vanilla, and beat until well combined.
Add flour mixture, alternating with buttermilk, scraping the sides of the bowl with a rubber spatula as needed.
In a small bowl, mix baking soda and vinegar until combined.
Add to batter, and beat for 10 seconds.
Evenly divide batter between the prepared pans.
Bake until a cake tester inserted in the center of each cake comes out clean, 20-25 minutes minutes.
Transfer to a wire rack to cool in the pans for 5 minutes. Remove from the pans, and return to the rack to cool completely.
In medium sauce pan add the egg whites and sugar.
On medium heat you are going to whisk egg mixture until sugar is completely dissolved (you can test it by rubbing a small amount between your fingers to test it, it should be completely smooth).
Transfer to mixing bowl and with whisk attachment whisk on high until stiff peaks form and mixture is completely cooled.
Switch to paddle attachment.
On high speed add the butter one piece at a time. Mix until well combined.
Add in the vanilla, continue mixing until completely mixed in.
Add in the violet food coloring and mix until fully combined.
Put a small dollop of frosting on cake board/cake plate so the cake sticks.
Put on the cake first layer, spread on layer of buttercream, smooth with offset spatula.
Repeat with next cake layer.
Place last layer of cake on top.
Crumb coat the cake, refrigerate.
Apply and smooth out final coat of buttercream on top and sides.
Pipe on top the decorative swirls.
Very steady and carefully hold cake at a slight angle and sprinkle the cake with the sprinkles on the sides.
Then sprinkle the top.
Refrigerate cake to set.
Let set at room temp for at least an hour before serving so the buttercream softens.