Make the Cake Layers

1. Prepare three 9-inch cake pans with parchment paper circles in the bottom and grease the sides. Preheat oven to 350°F (176°C).
2. Combine the flour, baking powder, baking soda and salt in a medium sized bowl and set aside.
3. Add the butter and sugar to a large mixing bowl and beat on medium speed until light in color and fluffy, about 2-3 minutes. Do not skimp on the creaming time.
4. Add the vegetable oil and vanilla extract and beat until well combined.
5. Add the eggs one a time, mixing until combined before adding the next.
6. Add half of the dry ingredients to the batter and mix until mostly combined. Slowly add the buttermilk and mix until combined. Add the remaining dry ingredients and mix until well combined and smooth, then fold in the sprinkles. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated. Do not over mix the batter.
7. Divide the batter evenly between the cake pans and bake for 25-30 minutes, or until a toothpick comes out with a few crumbs.
8. Remove the cakes from the oven and allow to cool for about 2-3 minutes, then remove to cooling racks to cool completely. The cakes will fall a bit as they cool and end up flat on top.

Make the Buttercream

1. Beat the butter until smooth and creamy.
2. Slowly add half of the powdered sugar and mix until smooth.
3. Add the vanilla extract, 4-5 tablespoons of water or cream and salt and mix until smooth.
4. Slowly add the remaining powdered sugar and mix until smooth. Add additional water or cream, as needed to get the right consistency of frosting.

Assemble the Cake

1. Place the first cake on a serving plate or a cardboard cake round. Spread a little more than 1 cup of frosting evenly on top of the cake. Add the second layer of cake and another cup of frosting. Top the cake with the remaining layer.
2. Frost the outside of the cake with the remaining vanilla buttercream. Refer to my tutorial for frosting a smooth cake, if needed. Use an offset spatula to create a spiral design on the top of the cake, if desired.
3. Press some additional sprinkles onto the sides of the cake and add some around the top edge.
4. Store the cake in an airtight container. It should be fine at room temperature for about 24 hours, then can be refrigerated. Serve at room temperature. Cake is best eaten within 3-4 days.