Preheat the oven to 300°F (148°C). Prepare three 8-inch cake pans with parchment circles and grease the sides.
Combine flour, sugar, cocoa, baking soda, and salt in a large mixer bowl and whisk together.
Add eggs, vegetable oil, buttermilk, and vanilla extract. Mix until smooth.
Slowly add hot water and mix until well combined.
Divide batter evenly between pans and bake for 30-33 minutes, or until a toothpick comes out clean.
Remove cakes from the oven and cool for 10 minutes, then turn out onto cooling racks to cool completely.