Preheat to 350 degrees. Line a 9x13 inch cake pan with parchment.
Sift together the flour, sugar, salt, baking soda, baking powder and cocoa powder into a large bowl and whisk to combine.
Add the sour cream, buttermilk, eggs, vanilla, and vegetable oil into a medium bowl and whisk together then pour in the coffee and mix once more.
Add the wet ingredients to the dry ingredient bowl.
Whisk to combine then mix on low for about a minute.
Pour batter into the lined pan. Bake for about 50 minutes or until a toothpick inserted in the center comes out clean.
Allow to cool for a few minutes then invert onto wire rack. Peel parchment paper off and let cool fully.
For the Frosting:
If you're using a nice runny fudge then have that ready. If you don't have that on hand then melt about 1/4 cup milk or cream with 1/4 cup dark chocolate, mix to combine then set aside to cool. Cream butter in a stand mixer fitted with a paddle attachment. Beat in cocoa powder and salt and mix until combined.
Add the confectioners sugar in a few batches mixing in on low. Scrape the bowl down and mix once more.
Add fudge or ganache in and mix until well combined. You can add milk or cream in a tablespoon at a time to thin the consistency out If needed.
For the Assembly:
Add buttercream to the top of the cake with an ice cream scooper. Spread over with an offset spatula.
Decorate with sprinkles, chopped toasted nuts or shaved chocolate
Video
Notes
If You Don't Have Fudge Handy
If you're substituting the fudge for a ganache just scald 1/4 cup of cream or milk and pour over an equal amount or semi or bittersweet chocolate chips (or chopped). Let it sit for a few minutes then whisk together. You can add more or less of this into the buttercream It's really up to you!
TIPS AND POINTERS:
Use whole milk with a tablespoon of vinegar or lemon juice as a substitute for the buttermilk.
You can substitute whole milk yogurt for the sour cream if desired, I do it all the time and can’t really tell the difference.