(adapted from "Pie in the Sky" by Susan Purdy)
13 3/4 oz all-purpose flour
2 1/4 teaspoons baking powder
3/4 teaspoon salt
1 cup butter, room temperature
2 cups, minus 1 tablespoon, sugar
4 eggs, room temperature
1 egg yolk, room temperature
2 teaspoons vanilla extract
1 cup, plus 2 tablespoons, buttermilk, room temperature
additional flour for dusting pans
1. Heat your oven to 350. Use a folded paper towel to generously grease 2 9-in round pans with vegetable shortening. Add 1 tablespoon all-purpose to each pan and shake it around to coat; tap out the excess. Line the bottom with a circle of parchment paper, if you want (this is a good way to make sure the cake doesn't stick to the bottom of the pan. Sometimes I use the parchment paper, sometimes I don't).
2. Sift together the flour, baking powder, and salt; set aside.
3. In a large bowl of a stand mixer, cream the butter and sugar together until well blended. Add the eggs and egg yolk (2 or 3 at a time), and vanilla, scraping down the bowl and mixing well after each.
4. Add the flour mixture and buttermilk in 2-3 batches, beginning and ending with the flour. Mix on low and scrape the sides of the bowl after each addition. Mix on medium for 1 minute, until the batter is smooth and creamy, and thickened up a bit.
5. Divide the batter evenly between the two prepared pans (I use my digital kitchen scale for this, too, so I'm sure my cakes will be the same height.) and spread it evenly. Bake at 350 for 28-30 minutes, or until a toothpick inserted in the center comes out clean or with just a few moist crumbs stuck to it. Cool on racks for 10 minutes before removing from the pans.
6. To remove the cakes from the pans, run a thin knife around the outside of the cakes to makes sure the sides have come completely away from the pans. Put a cooling rack over the pan, invert the cake onto the rack, remove the pan, and peel off the parchment. Invert the cake a 2nd time onto another rack, and let it finish cooling. (I've also used plates, cutting boards or foil-covered carboard cake circles to do this.) Cool the cakes completely before trimming the tops to make them level (if necessary) and frosting.
Makes 2 9-inch round cakes or 1 9x13-inch cake (You can use 2 8-inch round pans, but you have to make sure they are 2-inch deep pans, otherwise you'll have cake batter all over your oven. You will also need to bake the cakes a little longer. Start at 30 minutes and then check every few minutes until the cakes pass the toothpick test.)