Preheat oven to 350°F. Grease and flour two 9-inch round cake pans.
In a large bowl, whisk together flour, sugar, cocoa powder, baking powder, baking soda, and salt.
Add buttermilk, oil, eggs, and vanilla. Mix until smooth.
Slowly stir in hot coffee. The batter will be thin — that's what makes this chocolate cake extra moist.
Divide evenly between pans and bake 30–35 minutes, until a toothpick comes out with moist crumbs.
Cool completely before frosting.
For the frosting, beat butter until creamy. Add cocoa powder and mix well.
Gradually add powdered sugar, alternating with heavy cream, until thick and fluffy. Stir in vanilla and salt.
Frost between layers, then cover the top and sides with a generous layer of fudge frosting.
