Step 1: Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans or a 9×13 inch baking pan. Line the bottoms with parchment paper if using round pans.
Step 2: In a large bowl, whisk together the flour, sugar, cocoa powder, baking soda, baking powder, and salt until well combined.
Step 3: Add the buttermilk, vegetable oil, eggs, and vanilla extract to the dry ingredients. Beat with an electric mixer on medium speed for 2 minutes, scraping down the sides of the bowl as needed.
Step 4: Carefully stir in the hot coffee (or hot water) until just combined. The batter will be thin.
Step 5: Divide the batter evenly between the prepared cake pans.
Step 6: Bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean. If using a 9×13 pan, baking time might be slightly longer (35-40 minutes).
Step 7: Let the cakes cool in the pans for 10-15 minutes before inverting them onto a wire rack to cool completely. Once completely cooled, frost with your desired frosting.