Preheat the oven to 350F and grease and line the bottoms 3 – 9″ cake pans with parchment paper. You can also use 3 – 8″ cake pans but you will need to leave out about 1 cup of batter (If the pans are overfilled the cakes will sink in the middle. Make sure to only fill the pans a little over halfway). Line the pans in cake strips if you have them.
Whisk together the flour, baking powder, baking soda, and salt and set aside.
Using a hand or stand mixer, cream together the butter and sugar until light and fluffy mixing on medium high speed for 3-5 minutes with the whisk attachment. *see picture in post for reference
With the mixer running on low, pour in the oil. Scrape down the bowl and give it another mix.
Mix in the eggs one at a time followed by the vanilla. Scrape down the bowl after every 2 eggs.
Mix in the sour cream.
With the buttermilk and dry ingredients, mix into the batter alternating between the two, starting and ending with the dry ingredients (dry, wet, dry, wet, dry).
Take the bowl off the stand once the batter is combined. Take a rubber spatula and give it a good mix, scraping down the bottom and sides of the bowl. Then, fold in the sprinkles.
Pour into your cake pans and spread it even. For 9″ cakes, bake for 30-35 minutes. For 8″ cakes, bake for 35-40 minutes. They’re ready when a toothpick in the center comes out clean.
Allow the cakes to cool in the pans for 30 minutes, then run a knife around the edges and turn them out on to a cooling rack and peel the paper off. If you’re frosting the cake that day, transfer the cakes (still on the cooling rack) to the fridge to completely chill before assembling (about 1 hour). If baking the cakes a day ahead, allow the cakes to cool to room temp on the rack for 1 hour, then wrap in plastic wrap and place in the fridge overnight.
Mix the butter using a hand or stand mixer with the whisk attachment until smooth.
Mix in one cup of powdered sugar at a time, adding the vanilla and milk as it begins to thicken.
If the buttercream is still too thick when you’re done, add another tablespoon of milk.
Level off the chilled cakes if needed.
Spread a thin layer of buttercream between each cake layer, then give the entire cake a thin crumb coat. Do so by spreading a very very thin layer all around the cake, just enough to enclose all of the crumbs. Place the cake in the fridge for 20 minutes.
Frost the cake with the rest of the buttercream and decorate with extra sprinkles. Enjoy!