To make this recipe, you will need to preheat your oven to 350°F and grease two 9" cake pans or line muffin tins. You should also line the bottoms of the cake pans with parchment paper.
In a separate bowl, sift together the cocoa powder, flour, and salt. Set this mixture aside.
Using a large mixer fitted with the paddle attachment, beat the oil and sugar together on medium speed until they are fully mixed and slightly lightened in color. Then, turn the mixer to low and add the eggs one at a time, followed by the vanilla. Make sure to add each egg when the previous one is almost fully mixed in. Scrape the sides of the bowl and beat the mixture on medium speed until it is fully emulsified and smooth.
With the mixer on low speed, add the flour/cocoa mixture and the buttermilk to the bowl in two alternating additions: first the flour, then the buttermilk, followed by the remaining flour and buttermilk. Scrape the sides of the bowl and mix again on low speed until everything is well combined.
In a small bowl, whisk together the baking soda and white vinegar. This will cause a mini-volcano-like reaction. Add this mixture to the cake batter along with 1/4 cup of water (or food coloring, if you're using it) and mix on medium speed for about 10 seconds to fully combine. Scrape the sides of the bowl again.
Divide the cake batter evenly between the prepared cake pans or muffin tins and bake them in the center of the oven until they are done. The cake layers will take about 25 to 35 minutes to bake, while the cupcakes will take about 18 to 20 minutes.
When the cakes are finished baking, let them cool in the pans until you can handle them without an oven mitt, but they are still warm. Then, turn them out onto cooling racks, removing the parchment paper from the bottoms of the cakes. Allow the cakes to cool completely before eating or decorating.