Preheat the oven to 350°F (175°C). Grease and flour two 8-inch round cake pans. In a big bowl, mix flour, sugar, 1 cup of unsweetened cocoa powder (or use black cocoa powder for a darker color), baking powder, baking soda, and salt. Add buttermilk, vegetable oil, eggs, and vanilla extract, and mix until smooth. Stir in boiling water until well combined. Divide the batter into the two cake pans used.