1. Make the Chocolate Cake

• Preheat oven to 350°F (175°C). Grease and flour two 8-inch or 9-inch round cake pans. Line the bottoms with parchment paper rounds.

• Combine dry ingredients: In a large bowl, whisk together the flour, cocoa powder, baking soda, baking powder, salt, and granulated sugar. Break up any lumps and mix well.

• Combine wet ingredients: In a separate medium bowl, whisk together the eggs, buttermilk, oil, and vanilla extract.

• Combine wet and dry: Pour the wet ingredients into the dry ingredients and whisk until just combined. Do not overmix.

• Add hot water/coffee: Carefully pour the hot water or hot coffee into the batter and whisk on low speed until smooth. The batter will be thin.

• Bake: Divide the batter evenly between the prepared cake pans and bake for 30-35 minutes, or until a wooden toothpick inserted into the center comes out clean.

• Cool the cakes: Let the cakes cool in the pans for 10-15 minutes before inverting them onto a wire rack. Peel off the parchment paper and let cool completely.

2. Make the Creamy Filling

• Cream butter and shortening: In a large bowl using an electric mixer, beat the softened butter and shortening together on medium-high speed until light and fluffy (about 3-4 minutes).

• Add powdered sugar: Gradually add the sifted powdered sugar while mixing on low speed, then beat on medium until incorporated.

• Add marshmallow fluff: Add the marshmallow fluff, vanilla extract, and salt. Beat until combined.

• Adjust consistency: Increase speed to medium-high and beat for another 2-3 minutes until fluffy. If the filling is too stiff, add heavy cream or milk, one tablespoon at a time, until it reaches a smooth, spreadable consistency.

3. Assemble the Cake Base

• Level the cakes: Once the cakes are completely cool, use a long serrated knife to level the tops if they domed during baking.

• Layer the cake: Place one cake layer, cut-side up, on your serving plate. Spread the filling evenly over the layer, leaving a small border, or pipe a thick border of filling around the edge and fill the center.

• Top with second layer: Carefully place the second cake layer on top, cut-side down.

• Chill the cake: Refrigerate the assembled cake for 30-60 minutes. This helps the filling firm up for easier ganache application.

4. Make the Chocolate Ganache Coating

• Heat cream: Place the chopped chocolate in a heatproof bowl. Heat the heavy cream in a saucepan over medium heat until it just begins to simmer.

• Combine cream and chocolate: Pour the hot cream over the chopped chocolate and let it sit for 5 minutes, then whisk gently until smooth.

• Add optional ingredients: Stir in the corn syrup and room-temperature butter for extra shine and richness.

• Cool the ganache: Let the ganache cool slightly for 10-15 minutes until it’s pourable but not hot.

5. Coat the Cake

• Prepare for ganache: Place the chilled cake on a wire rack set over a large baking sheet to catch any drips.

• Pour ganache: Slowly pour the ganache over the top of the cake, allowing it to flow down the sides. Use an offset spatula to spread it evenly if needed.

• Let the ganache set: Allow the ganache to drip for a few minutes before moving to the next step.

6. Final Chill and Serve

• Chill the cake: Carefully move the cake back to the refrigerator for at least 1-2 hours, or until the ganache is completely set.

• Decorate: Optionally, add a decorative swirl using leftover filling or white chocolate.

• Serve: For cleaner slices, dip a sharp knife in hot water and wipe it clean between each cut. Let the cake sit at room temperature for about 20-30 minutes before serving for the best texture.