Preheat oven to 350°F. Grease a 9-inch round cake pan with butter and line bottom with parchment paper.
In a medium bowl, whisk together flour, cocoa powder, baking soda, and salt. Set aside.
In a large bowl, beat together sugar, oil, eggs, and vanilla until light and fluffy. Add dry ingredients to wet ingredients in two batches, mixing until just combined.
Add buttermilk and mix until combined. Pour batter into prepared pan and bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
Allow cake to cool completely before serving.
Interesting Facts
This cake can also be made with oil or melted butter instead of vegetable oil.
The cake can be stored in an airtight container for up to 4 days.
This cake can also be made in a 9x13-inch pan for a thicker cake.