Preheat the oven to 350°F. Grease and line three 4-inch cake pans or one 6-inch cake pan.
Place all of the dry ingredients into a medium bowl and whisk them together. Add the egg, vegetable oil, vanilla, and buttermilk and whisk until fully combined. Add the hot coffee (or hot water) in a slow stream and whisk until fully combined and smooth. The batter will be very thin.
Pour the batter into the prepared cake pan(s). If using 4-inch cake pans, bake for 18-22 minutes. If using a 6-inch cake pan bake for 24-28 minutes.
