Preheat oven to 325F. Grease and flour two 8" cake rounds and line with parchment.
In a medium bowl, whisk flour, baking powder, and salt until well combined. Set aside.
Using a stand mixer fitted with a paddle attachment, beat butter until smooth and pale. Add sugar and oil and beat on med-high until pale and fluffy (approx 3mins). Reduce speed and add egg whites one at a time fully incorporating after each addition. Add clear vanilla.
Alternate adding flour mixture and milk, beginning and ending with flour (3 additions of flour and 2 of milk). Fully incorporating after each addition.
Bake for approx. 35mins or until a toothpick inserted into the center comes out mostly clean.
Place cakes on wire rack to cool for 10mins then turn out onto wire rack.
Place egg whites and sugar into the bowl of a stand mixer, whisk until combined.*
Place bowl over a pot with 1-2" of simmering water and whisk constantly until the mixture is hot and no longer grainy to the touch or reads 160F on a candy thermometer (approx. 3mins)
Place bowl on your stand mixer and whisk on med-high until the meringue is stiff and cooled (the bowl is no longer warm to the touch (approx. 5-10mins)).
Switch to paddle attachment. Slowly add cubed butter and mix until smooth.**
Add clear vanilla and whip until smooth.
Add violet color gel and/or bright white color gel as needed.
Place one layer of cake on a cake stand or serving plate. Top with approximately 1 cup of buttercream. Repeat with remaining layer and crumb coat the cake. Chill for 20mins.
Frost the top and sides of the cake and smooth with a bench scraper.
Pipe rosette dollops using a 1M tip with remainder of frosting.
Chill cake until frosting is firm. Gently press sprinkles gently along the bottom of the cake and sprinkle along the top.
* Wipe your mixer bowl and whisk down with lemon juice or vinegar to make sure it is completely grease free and make sure there is no trace of yolk in your whites or your meringue will not stiffen.** The buttercream may look like it's curdled at some point. Keep mixing until it is completely smooth. If it looks soupy, place it in the fridge for 20mins and rewhip.