Preheat your oven to 350 degrees Fahrenheit (175 degrees Celsius) and grease a 9-inch by 13-inch rectangular cake pan with butter, lining the bottom with parchment paper.
In a large bowl, combine the sugar, flour, cocoa powder, baking powder, baking soda and salt with a wire whisk.
Measure the buttermilk and vegetable oil into a large liquid measuring cup.
Add the eggs, vanilla and vinegar to the milk mixture, along with the red food colouring, and whisk all the wet ingredients together with a fork.
Add the wet ingredients to the dry ingredients and whisk together just to combine (until no streaks of flour remain and the batter is a smooth, even red colour).
Slowly add the boiling water, whisking constantly until the batter is smooth and everything is combined.
Pour the batter into the prepared baking pan and bake for about 30-35 minutes or until the a toothpick inserted into the middle of the cake comes out clean.
Let cool on a wire rack in the pan until cooled completely to room temperature.