Preheat oven to 350F. 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, cream the butter and sugar on med-high until pale and fluffy (about 3mins). Reduce speed and add eggs one at a time fully incorporating after each addition. Add vanilla.
Alternate adding flour mixture and buttermilk, beginning and ending with flour (3 additions of flour and 2 of milk). Fully incorporating after each addition.
Bake for 30-35mins 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 vanilla and whip until smooth.
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.
Mix confetti and nonpareils in a small bowl. Press sprinkles gently along the bottom of the cake and sprinkle along the top. For the speckled sides, I grabbed a pinch of sprinkles and tossed them randomly at the sides.
Pipe rosettes using a 1M tip with remainder of frosting.
Be sure to check my Swiss Meringue Buttercream post for tips and troubleshooting.* Ensure there is NO trace of egg yolks in your whites and that your mixer bowl and whisk is completely grease-free or your meringue won't stiffen.** The buttercream may look like it's curdled at some point. Keep mixing until it is completely smooth.*** I used the left side of the one on the left pictured here.