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.