Spray a 9×13 baking pan with cooking spray. Preheat the oven to 350 degrees Fahrenheit.
For the cake, put flour, sugar, cocoa, baking powder, baking soda and salt in a bowl and mix well.
In a stand mixer, mix the buttermilk, oil, eggs, vanilla together. Slowly add in the mix of dry ingredients while the mixer is on low. Add the coffee and make sure everything is combined. The batter will be very thin.
Pour the batter into the prepared pan and bake for about 30 minutes or until the center of the cake springs back when you touch it.
MAKE THE FROSTING:
In a small saucepan, heat the brown sugar, pecans, butter and heavy cream.
Heat over low heat, melt the butter and bright to a simmer. Boil lightly for two minutes, then remove from the heat and stir in the vanilla.
Add the powdered sugar and mix well.
Spread the frosting over the cake while the cake is cool but the frosting is still warm. It will harden as it sits so work with the frosting quickly.