Step 1: Preheat your oven to 350°F (175°C). This ensures that your cake bakes evenly.
Step 2: Grease your two 9-inch round cake pans with butter or cooking spray. Line the bottoms with parchment paper.
Step 3: In a large mixing bowl, combine the softened unsalted butter and granulated sugar. Beat on medium speed until light and fluffy, about 3-5 minutes.
Step 4: Add the eggs and egg yolks one at a time, mixing well after each addition. Scrape down the sides of the bowl to ensure everything is well combined.
Step 5: In a separate bowl, whisk together the all-purpose flour, baking powder, and kosher salt. Gradually add this mixture to the butter and sugar mixture, alternating with the buttermilk. Start and end with the dry ingredients, mixing until just combined.
Step 6: Stir in the vanilla extract, mixing until smooth. Be careful not to overmix; a few lumps are okay.
Step 7: Divide the batter evenly between the prepared cake pans. Bake for 25-30 minutes, or until a toothpick inserted comes out clean.
Step 8: Once baked, cool the cakes in the pans for about 10 minutes. Transfer to a wire rack to cool completely before frosting.