Line the bottom of two 9-inch round cake pans with buttered parchment paper. Position rack in the middle of oven and preheat to 350°F.
Soak the Bake-Even Strips in cold water for 5 minutes, then rap around the outside of the cake pans.
For the cake: Stir together the flour, baking soda, and salt in a bowl, mixing well.
Place the sugar and butter in a large bowl and beat on medium speed for about 5 minutes, or until very soft and light. Beat in the eggs one at a time, beating well after each addition, then beat in vanilla.
Reduce the speed to low and beat in ⅓ of the flour mixture, then ½ the buttermilk (or sour cream/water mixture), stopping and scraping down the bowl and beater after each addition. Beat in another third of the flour, then the remaining buttermilk, stopping and scraping again. Finally, beat in the remaining flour mixture.
Scrape the bowl well with a large rubber spatula.
Pour the batter into the prepared pans and smooth the tops.
Bake the layers for about 30 to 35 minutes, until they are firm and a toothpick inserted in the center emerges clean. Cool the layers in the pans on racks for 5 minutes, then unmold onto racks to finish cooling before icing.
Trim the top of each cake layer to flatten, if needed. You won't need to trim them if you follow my advice about the Bake-Even Strips...I'm not kidding, they really work!
For the icing: Stir together powdered sugar, corn syrup, lemon juice, vanilla, and 1 tablespoon water in a small bowl until smooth.
Transfer half of icing to another bowl and stir in cocoa powder and espresso powder (if using), adding more water, ½ teaspoon at a time, to thin to same consistency as white icing.
Finish the cake: Ice the bottom layer half white and half black. Place the top layer on top using cake lifters.
Frost the opposite sides of the top layer white and black so you’ll get both black & white icing in every bite. Serve & enjoy!
If, like me, you don't keep buttermilk at hand, there are a few substitutions you can try. For 1 cup of buttermilk in a recipe:
Use ½ cup each water and sour cream (or plain Greek yogurt). Stir well.
For 1 cup of milk, remove 1 teaspoon and replace it with 1 teaspoon lemon juice or white vinegar. Let sit for 5 minutes before using.
Use 1 cup of water and 2 tablespoons sweet buttermilk powder. This is my method of choice, and what I use in the video. Just whisk the buttermilk powder in with the flour mixture.
Use a light hand with the lemon juice. You want the taste to be barely noticeable. It’s to cut the overwhelming sweetness of the icing without making it taste too lemony.Adding espresso powder to the chocolate icing deepens the chocolate flavor. It's optional, but totally worth it!This recipe makes one 9-inch cake, but you can easily half the amounts to make a 6-inch cake using two 6-inch round cake pans.