Preheat your oven to 350°F (175°C). This ensures that your cake bakes evenly.
Grease and flour two 9-inch round cake pans. This will help your cakes release easily after baking.
In a large mixing bowl, beat the softened butter and granulated sugar together using an electric mixer on medium speed until light and fluffy, about 3-4 minutes.
Add in the eggs, one at a time, mixing well after each addition. Then, add the egg yolks and mix until fully incorporated.
In a separate bowl, whisk together the all-purpose flour, baking powder, and kosher salt.
With the mixer on low speed, add the dry ingredients to the butter mixture in three additions, alternating with the buttermilk in two additions. Start and end with the dry ingredients. Mix until just combined; do not overmix.
Gently fold in the vanilla extract until evenly distributed throughout the batter.
Evenly divide the batter between the prepared cake pans.
Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out clean.
Remove the cakes from the oven and let them cool in the pans for 10 minutes. Then, carefully remove the cakes from the pans and transfer them to a cooling rack to cool completely.