• Preheat the oven to 180°C (350°F) and prep two 8-inch baking pans with cooking spray and parchment paper.
• Into a large mixing bowl, add the eggs, buttermilk, oil, vanilla extract, kosher salt, granulated sugar, sour cream, and sift the cocoa powder, and mix with a whisk until well combined.
• Sift the flour, add the baking powder, and baking soda, and mix until almost combined.
• Add the hot coffee and mix until the batter is smooth and there are no large lumps of flour visible.
• Divide the mixture into two 8-inch baking pans, I suggest using cake strips for even baking. (Weigh the batter for better results.) and bake in a preheated oven for 25-30 minutes or until a toothpick comes out clean. (You can also use a 9-inch cake pan.)
• Allow the cake layers to cool completely. Run a butter knife or a thin spatula around the edges of the cake to release it from the sides of the pan.
• Invert and Tap: Place a wire rack or plate over the pan, hold them firmly, and flip to invert the pan. Lift Gently: Carefully lift one side of the pan, then remove it completely to slide your cake onto the surface.
• Once the two cake layers are completely cool, use a long serrated knife to carefully slice each one in half horizontally, creating four even layers for filling with the mousse.
• In a small bowl, add the water and the gelatin, and let it sit for 10 minutes to allow the gelatin to bloom.
• Melt the dark chocolate and heavy cream together in a medium mixing bowl. You can do this in the microwave using 10–20 second bursts, stirring between each, or use the bain-marie (double boiler) method for gentle, even melting.
• Mix the melted chocolate and heavy cream with a whisk or with an immersion blender to ensure it's smooth and the emulation is perfect.
• In a large mixing bowl, add the cold heavy cream, sifted cocoa powder, powdered sugar, and vanilla extract, and whip to soft peaks, resembling a yogurt consistency.
• Melt the gelatin in the microwave for 5-second pulses until it's fully melted. Do not overheat it! Add the melted gelatin to the melted chocolate and heavy cream mixture and mix until well combined.
• Add about ½ cup of the mousse mixture into the melted chocolate mixture and mix it until smooth and combined.
• Add half of the chocolate mixture to the whipped cream mixture and gently fold it with a rubber spatula until it's mostly combined.
• Add the other half and fold it until the mixture is smooth and homogeneous and doesn’t have streaks of chocolate. Try not to fold it too much to retain the air in the whipped mixture.

Assembly

• Place one cake layer on a serving plate or cake board. Spread a layer of chocolate mousse evenly over the first layer. Add the second cake layer on top, then spread more mousse. Repeat with the third layer.
• Place the fourth cake layer on top and frost the entire cake with the remaining mousse, sides, and top. Place the cake in the fridge to set while you make the chocolate ganache.
• Heat the heavy cream in a small saucepan over medium heat until it’s just about to simmer (don’t let it boil). Place the chopped chocolate in a heatproof bowl.
• Pour the hot cream over the chocolate and let it sit for 2–3 minutes. Gently stir until the chocolate is fully melted and the ganache is smooth and glossy.
• Let it cool slightly until it’s pourable but not too runny, then pour it over the chilled cake and let it drip down the sides.
• Pour the chocolate ganache over the cake and use a spatula to carefully push the ganache around the edges of the cake until a drip design forms.