• Preheat the oven to 350℉. Grease or spray a 9X13 pan with baking spray or grease two or four 8-inch or 9-inch cake pans. Then line with parchment rounds, and grease the parchment paper. Parchment paper will help release the cake from the pan. If you would like to remove the 9 X 13 cake, I recommend using parchment paper as well. Set aside.

Make the Chocolate Cake

• Make the cake: (Dry Ingredients) Whisk flour, sugar, cocoa powder, baking soda, baking powder, and salt together in a large bowl.

  (Wet Ingredients) In a large bowl or stand mixer on medium-high speed, mix the oil, sour cream, eggs, and vanilla until combined. Then add the buttermilk and mix until combined.

  Add the dry ingredients to the wet ingredients and mix until combined. Add in hot coffee or water and beat until the batter is combined.
• Pour the batter into a prepared 9x13-inch baking pan, or divide it evenly between two or four 8- or 9-inch round pans.

  For a 9x13 pan, bake for 30 to 40 minutes, or 4 cake layers, bake for 19 to 23 minutes. You can also bake 2 round cakes for 23 to 26 minutes and cut them in half to create 4 layers.

  Keep an eye on your cakes—baking times can vary. You'll know they’re ready when a toothpick inserted in the center comes out clean.
• Remove cake from oven and set the pan/s on wire racks to cool completely in the pan.

Prepare Chocolate Mousse Filling / Topping: (Chill time needed: 2 hours up to 2 days ahead)

• Whisk hot water and cocoa powder together in a bowl.
• Place chopped chocolate bars in a double boiler to melt. You can also melt them in the microwave by placing them in a heat-proof bowl and melting them 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 the chocolate mixture into the whipped cream until combined. Do not overmix or mix too hard so that the mousse does not deflate. Cover and refrigerate 2 hours or up to 2 days until chilled.

Prepare Chocolate Ganache

• As the cakes cool, prepare the Ganache (about 25 minutes ahead of adding it to the cake)

  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 before spreading on chilled cake.

Simple 9 X 13 Layer Cake

• For a 9x13 Cake: There are two ways you can layer this cake

  1 - Cake / Ganache / Mousse: First, you can pour the ganache over the fully cooled chocolate cake to create a distinct ganache layer, refrigerate for 1 hour, then spread the whipped mousse on top.

  Or, if you prefer, pour the ganache over the cake while it’s still slightly warm so it soaks in a bit, refrigerate for 1 hour, then add the mousse once it’s cooled (as pictured in the post).

  2 - Cake / Mousse / Ganache: Alternatively, you can reverse the order—spread the mousse over the cake first and drizzle the ganache on top, just like with the round layer cake version. Then garnish.

To Assemble Round Cake Layers

• Chocolate Mousse Layer: Once the cake is completely cooled, layer it with chocolate mousse.

  *For Layer Cakes: To assemble the cake with the whipped chocolate mousse icing, start by leveling your cake layers, if needed. If the cakes are domed, use a large serrated knife or a cake leveler to carefully slice off the rounded tops and create flat, even surfaces.

  For a 4-layer cake, divide the mousse into three equal portions—about 1½ cups each. Spread one portion between each of the first three layers, top with the 4th cake, reserving a spoonful to lightly coat the sides for a crumb layer.

  Lightly coat the sides for a crumb layer by smoothing with a spatula or bench scraper, before topping with ganache.

  Refrigerate cake for at least 1 hour and up to 4 hours. Prepare the ganache as you wait about 25-30 minutes before adding the ganache.
• Chocolate Ganache Layer: Pour or spoon ganache on chilled cake. Smooth the ganache with a spatula or a bench scraper.

Garnish

• Garnish with chocolate curls, sprinkles, chocolate chips or raspberries or strawberries.

Serve / Store

• Cake can be served immediately or chilled uncovered for 4–6 hours before serving.
• Cover leftover cake and store in the refrigerator for up to 5 days.