Preheat the oven to 375 degrees F. Spray two 9-inch round cake pans with nonstick spray. I like to use the Pam Baking Spray and line them each with a round of parchment paper too.
In a large bowl, use an electric mixer to combine the butter and sugar. Mix until light and fluffy- about 2 minutes. Beat in the eggs until well incorporated, scraping down the sides of the bowl as needed. Mix in the buttermilk. In a separate bowl, whisk together the flour, cocoa powder and baking soda. Then mix in the dry ingredients a little at a time until all is mixed well. Add the vanilla and the hot water, and continue to mix until the batter is well-incorporated and smooth. Divide the batter between the two prepared pans. Bake about 25 minutes, or until the center is set and a toothpick inserted comes out nearly clean. Set aside to cool for at least 20 minutes, then gently turn over onto racks to cool the cake layers completely before frosting.
Frost when cool, or wrap in plastic wrap until ready to frost (cake layers can be made one day ahead).
Notes
Nutritional information does not include frosting.
*For high altitude (above 5,000 feet), reduce sugar by 1/4 cup, reduced baking soda by 1/2 teaspoon, and add an additional tablespoon of hot water.
*I like to use bake even Cake Strips to ensure that I get nice flat cake layers. They work great.