Preheat the oven to 350°F. Grease and flour a 13x9 inch cake pan.
In a large mixing bowl or the bowl of a mixer with the paddle attachment, combine the sugar, vegetable oil, and butter. Cream together at medium speed for 1 minute.
Add the eggs and egg yolks one at a time on low speed. Stir in the vanilla extract until the batter is smooth and incorporated, scraping down the sides of the bowl as needed. Next, mix in the sour cream and beat until well-combined.
In a separate bowl, whisk the flour, baking powder, salt, and turmeric until thoroughly combined.
Add half the flour mixture, and beat on low speed while pouring in half of the buttermilk and mix just until the flour starts to incorporate. Finally add the remaining flour, mixing on low speed while pouring in the remaining buttermilk and beating until all of the ingredients are combined.
Use a spatula to scrape down the sides of the bowl and stir the batter from the bottom to the top to ensure it’s well-mixed.
Pour the batter into the greased pan (or pans) evenly and bake at 350°F for 36-40 minutes (or less time for cupcakes), keeping a careful eye on the baking during the last few minutes. Test the cake for doneness by inserting a toothpick into the center of the cake––if it comes out clean, the cake is done.
Cool completely and decorate with your choice of frosting.
Note: Nutrition information is for cake only, without frosting.