Preheat oven to 350F and prep 3 9-inch round baking pans by greasing the bottom and sides. Additionally (and this is optional), you can line the bottom with parchment paper to ensure easy release.
In a large bowl, whisk flour, baking powder, baking soda and salt. Set aside.
Using a handheld electric mixer or a stand mixer fitted with the paddle attachment, beat butter on medium-high speed until smooth, light and creamy (about 2 minutes).
Add sugar and beat on medium-high speed for another 5-7 minutes until very creamy (almost like whipped cream - you cannot see grains of sugar anymore). Remember to scrape the bottom and sides of your bowl.
Add the eggs one at a time until each one is well incorporated. Then add vanilla.
Switch to low speed and add a third of your flour mixture. Then half of your buttermilk. Another third of your flour. The last of your buttermilk then finally the last third of your flour. Make sure each addition is mixed just until incorporated before the next one is added (do not over mix). You will notice the batter will be thick but still smooth and creamy.
Turn to your egg whites and beat until thick and foamy. Gently fold into your batter. Then gently fold sprinkles.
Transfer your batter into your prepared pans and bake for 25 minutes or until a toothpick inserted in the middle of the cake comes out clean.
Cool the cakes completely on a wire rack before frosting.
For the Frosting:
Using a handheld electric mixer or a stand mixer fitted with the paddle attachment, beat butter on medium speed until creamy (about 2 minutes).
Switch to low speed and add heavy cream, vanilla, salt and confectioner's sugar (don't add the sugar all at once though; do it gradually, about ½ cup at a time).
Once everything is incorporated switch to medium-high speed and beat the frosting until you get the consistency you desire. You might need to add more sugar if the frosting is too watery, more cream if the frosting is too thick or more salt if the frosting is too sweet.
Assembly:
Trim the tops of your cakes if they are uneven using a serrated knife or a cake leveller. The important thing is all three cakes are of equal height.
Take one and place on a platter or cake turntable, smooth side down (trimmed side up). Evenly cover the top with frosting. Repeat with the 2nd and 3rd layers. For your 3rd layer, remember to place it smooth side up (trimmed side down) so that you'll have an even top.
Finish the cake by also covering the sides of your cake with frosting. To help even out the frosting on top and the sides, use a bench scraper and gently scrape away excess frosting.
Decorate with sprinkles as desired.
Notes
For those of you who have little mixers like me - note that this is a lot of batter. My mixer almost overflowed and I had a few flour explosions so watch out.
I beat my 2 egg whites by hand and it was the most painful 5 minutes of my life (I have a brand new respect for bakers who whisk eggs by hand all the time). I haven't whisked eggs by hand again after this. I always turn to my trusty electric hand mixer.