Preheat oven to 350 and grease and flour a 9x13 baking pan. Set aside.

Add coffee and cocoa to a large mixing bowl and mix well to combine.

Add oil.

Add buttermilk and beat until smooth.

Beat in baking soda for 30 seconds and add eggs and vanilla. Beat until combined. Add sugar and flour and beat for 2 minutes at medium speed.

Pour batter into prepared pan.

Bake for 28-32 minutes or until a toothpick inserted in the center comes out clean and the edges just begin to pull away from the pan.

Remove from oven and cool for 30 minutes before putting in the freezer for one hour.

Beat together butter, salt, and vanilla until pale and fluffy, about 3 minutes.

Add half and half and sugar until you have a smooth consistency.

Place into a large piping bag and snip the tip to about 1/2" in diameter.

Pipe buttercream across the short edge of the cake, keeping the bumps about 1 inch apart. Freeze for an additional 30 minutes.

Add buttermilk, corn syrup, butter, cocoa and salt in a large saucepan. Heat over medium heat, whisking frequently until smooth. Bring mixture to a boil and boil for 2 minutes, until the bubbles snap and pop. Reduce heat to low and whisk in powdered sugar and vanilla until smooth.

Remove from heat let stand for 5 minutes to cool slightly.

Store any uneaten cake covered and in the refrigerator.