Preheat the Oven: Preheat your oven to 350°F (175°C). Grease and flour an 8-inch round cake pan, or line it with parchment paper for easy removal.
Mix the Cake Batter: In a large mixing bowl, whisk together the flour, sugar, cocoa powder, baking powder, baking soda, and salt. Add the vegetable oil, buttermilk, egg, and vanilla extract. Stir until a smooth batter forms. Gradually add the hot water (or brewed coffee) and mix until combined and smooth.
Bake the Cake: Pour the batter into the prepared cake pan, smoothing the top. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.
Prepare the Fudge Frosting: In a medium saucepan, melt the butter over low heat. Stir in the cocoa powder and heavy cream, then bring the mixture to a simmer. Remove from heat and whisk in the powdered sugar, vanilla extract, and a pinch of salt until the frosting is smooth and glossy.
Assemble the Cake: Once the cake has cooled, pour the fudge frosting over the top, spreading it evenly with a spatula. Allow the frosting to drip down the sides for a luxurious finish. Let it set for 10 minutes before slicing and serving.