Make the cake

Preheat oven to 350 degrees F. Grease or spray 9X 13 pan with baking spray. Set aside.

Whisk flour, sugar, cocoa powders, baking soda, baking powder and salt together in a large bowl.

In a large bowl or stand mixer, mix the buttermilk, oil, sour cream, eggs and vanilla together until combined.

Add in dry ingredients and mix until combined. Add in hot coffee or water and beat until the batter in combined.

Pour into prepared 9 x 13 baking pan and bake for 30 to 40 minutes. Keep an eye out as baking times vary. The cake will be done when a toothpick inserted into the center of the cake comes out clean. (For a layered cake instructions see notes*)

Remove cake from oven and let cool completely on wire racks.

Prepare chocolate mousse filling / topping

As the cake cools, prepare the mouse so it can chill and be ready.

Whisk hot water and cocoa powder together in a bowl.

Place chopped chocolate bars in a double boiler to melt. You can also melt in the microwave by placing in a heat proof bowl and melting in 20 second increments. Stir after each increment until melted and smooth.

Pour melted chocolate into hot cocoa water mixture and stir in with silicone spatula until combined and smooth in texture.

In a stand mixer or with hand mixer. Beat heavy whipping cream, powdered sugar, and vanilla together on medium-high speed until medium peaks form. About 3 to 4 minutes.

Using a silicone spatula, gently fold chocolate mixture into the whipped cream until combine. Do not over mix or mix too hard so that the mousse does not deflate. Cover and refrigerate 2 hours until chilled (you can make this ahead of time up).

Prepare Chocolate Ganache

Place chopped chocolate in a medium heat proof bowl.

Heat cream in a small saucepan over medium heat until it begins to simmer gently (do not bring to a rapid boil). Pour over the chocolate and let it stand for 2 to 3 minutes to soften the chocolate. With a spatula stir until the chocolate is completely melted and smooth.

Let it cool at room temperature for 20 minutes. *If making layered cake and not 9 X 13 see notes for layered cake instructions.

Once the mixture is cooled pour over 9 X 13 cake and refrigerate for 1 hour.

After refrigerating spread mousse over the ganache layer and serve or refrigerate until serving.

Garnish with chocolate curls or sprinkles if desired.

Cover leftover cake tightly and store in the refrigerator for up to 5 days.