Preheat oven to 350 degrees. Line 3, 8" round baking pans with parchment paper. Butter and flour pans (I use Baking PAM for this).

Use electric mixer with paddle attachment to beat butter, sugar and brown sugar, until lightened in color and it has a fluffy appearance, about 1.5 minutes.

Add eggs, one at a time, scraping down sides as needed, until well incorporated. Next, beat in vanilla extract.

In a medium sized bowl, stir together flour, corn starch, baking powder, and salt. Alternate beating flour mixture and buttermilk into butter mixture, in about 3-4 additions. Start and end with flour mixture. Scrape down sides with spatula as needed.

Stir in sprinkles.

Evenly divide batter among the 3 prepared pans. Bake until tops are golden brown and a toothpick inserted into center of cake comes out with just a few crumbs, about 28-32 minutes.

Let cook in pans for about 10 minutes, then transfer to cooling rack to cool completely before assembling.

Level cakes with a serrated knife or cake leveler.

Use electric mix to beat butter and shortening until lightened in color, and very smooth, about 2-3 minutes. Next beat in salt and vanilla extract.

Add powdered sugar, about 1/2 cup at a time. Beat until well combined, about 30 seconds. After each 1 - 1.5 cups of powdered sugar has been added, beat in about 1 tbsp of cream or milk. Do this until you reach your desired taste and consistency. If you prefer a more firm buttercream, you'll use more powdered sugar and less milk, and vice versa if you want a softer buttercream. NOTE: More firm buttercream works well for piping. I generally prefer 5 cups of powdered sugar and 4 tbsp of whipping cream.

Use buttercream to fill inside of cake. Stack cakes, with the leveled side down for the top layer. Apply a thin layer around entire cake which will serve as the crumb layer. Decorate as desired (see text above for details about how I decorated the cake in these photos).