Preheat oven to 325 degrees and spray a bundt pan with baking spray.
Beat butter, shortening, and sugar until light and fluffy, about 4 minutes.
Beat in eggs one at a time, stopping several times to scrape down the sides of the bowl.
Sift together flour, cocoa powder, baking powder and salt. Set aside.
Combine buttermilk, coffee, and vanilla.
Add flour mixture, alternating with buttermilk mixture, beginning and ending with flour and adding buttermilk in 2 additions. Beat just until incorporated.
Transfer batter to prepared pan, spreading it evenly. Bake for 1 hour and 20 minutes or until toothpick inserted in center comes out clean.
Let cool 10 minutes in pan and then remove from pan to cool completely.
To make frosting, bring heavy cream and brown sugar to a rolling boil in a medium saucepan. Boil for 1 minute.
Stir in baking soda and boil 1 more minute.
Remove from heat and add butter but do not stir. Pour into a large bowl. Let cool 5 minutes. Stir and then place in the refrigerator for 15 to 20 minutes, stirring occasionally.
Beat with a hand-held mixer or a whisk for about 1 minute. If it still is not thick enough, refrigerate until it is thicker, checking every 5 minutes. Pour on cake.