Vanilla Cake:
Preheat your oven to 350°F (175°C). Grease and flour two 8-inch cake rounds, then line them with parchment paper.
In a medium bowl, whisk together the 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 medium-high speed until pale and fluffy (about 3 minutes). Reduce the speed and add the eggs one at a time, fully incorporating each before adding the next. Add the vanilla extract.
Alternate adding the flour mixture and buttermilk to the butter mixture, starting and ending with the flour (3 additions of flour and 2 of milk). Mix each addition until fully combined.
Divide the batter evenly between the prepared cake pans and bake for 30-35 minutes, or until a toothpick inserted into the center comes out mostly clean.
Cool the cakes in the pans on a wire rack for 10 minutes, then turn them out onto the wire rack to cool completely.
Vanilla Buttercream:
Place the egg whites and sugar in the bowl of a stand mixer and whisk until combined.
Place the bowl over a pot with 1-2 inches of simmering water and whisk constantly until the mixture is hot and no longer grainy to the touch, or until it reaches 160°F on a candy thermometer (about 3 minutes).
Place the bowl on the stand mixer and whisk on medium-high speed until the meringue is stiff and the bowl is cool to the touch (about 5-10 minutes).
Switch to the paddle attachment and slowly add the butter, one cube at a time, mixing until smooth.
Add the vanilla extract and continue mixing until the buttercream is smooth and silky.
Assembly:
Place one cake layer on a cake stand or serving plate. Spread approximately 1 cup of buttercream on top.
Add the second layer and apply a thin crumb coat of frosting over the entire cake. Chill the cake for 20 minutes.
Frost the top and sides of the cake with the remaining buttercream, using a bench scraper to smooth the surface.
If desired, use a decorating comb to add texture to the sides of the cake.
In a small bowl, mix the confetti sprinkles and nonpareils. Gently press them along the bottom edge of the cake and sprinkle along the top. For a speckled effect, grab a pinch of sprinkles and toss them randomly at the sides of the cake.
Pipe rosettes using a 1M tip with any leftover frosting for decoration.