Preheat your oven to 300°F and line two 12-count cupcake pans with liners, and set aside.
In a medium bowl, whisk together 2¾ cups cake flour, ½ teaspoon baking soda, 1½ teaspoons baking powder, and ¾ teaspoon salt. Set aside.
In a large bowl with a hand mixer or stand mixer fitted with a whisk attachment, cream together 1½ cups granulated sugar, ⅔ cup salted butter, ½ cup sour cream, ⅓ cup vegetable oil, and 1 tablespoon vanilla extract.
Add the 2 large eggs and 2 large egg yolks one at a time, beating after each addition just until combined.
Using a rubber spatula, fold in the dry ingredients in 3 parts alternating with the 1 cup buttermilk in 2 parts. Begin and end with the dry ingredients. Gently fold until only very small lumps remain. DO NOT use an electric mixer for this step.
Evenly divide the batter between the cupcake liners, about scant ¼ cup of batter per liner, and bake for 25 to 30 minutes. A toothpick should come clean from the center when done.
Allow the cakes to cool in the pan for 5 minutes before removing and transferring them to a wire rack to cool completely (about 1 hour before frosting).