Preheat your oven to 350°F (180°C) and prepare a 9-inch springform pan by lining the bottom with parchment paper and spraying the sides with cooking spray. This will make the cake easy to remove later.

In a large mixing bowl, whisk together the eggs, buttermilk, vegetable oil, vanilla extract, and sour cream to keep the cake moist and tender. 

Add the granulated sugar and continue whisking until everything is well combined. 

Sift in the Dutch-processed cocoa powder to avoid lumps, then add the flour, baking powder, baking soda, and sea salt. Mix gently until everything is just combined, don't overdo it!

Now, pour in the hot brewed espresso. This adds a deep, rich chocolate flavor without making it taste like coffee. Stir until the batter is smooth and glossy. 

Pour the batter into your prepared pan and bake for 22-25 minutes, or until a toothpick inserted into the center comes out with just a few moist crumbs. 

Ensure the cake has completely cooled before layering the mousse.

To make the mousse, start by blooming the gelatin. Combine the gelatin powder and water in a small bowl and let it sit for about 10 minutes. 

Meanwhile, melt the dark chocolate and heavy cream in a heatproof bowl, either in the microwave in short bursts or over a double boiler. After melting, let it sit for a moment to cool slightly.

In another bowl, combine cold heavy cream, sifted cocoa powder, powdered sugar, and vanilla extract. 

Whip it to soft peaks, like creamy yogurt. Melt the bloomed gelatin in the microwave for a few seconds, then stir it into the melted chocolate mixture to create a smooth base.

To keep the mousse light, mix a small amount of the whipped cream into the chocolate to ease the incorporation. 

Then, gently fold in the rest of the whipped cream until the mixture is smooth and fluffy.

Once the cake has cooled, level it with a serrated knife if needed. Line the inside of the springform pan with acetate or parchment paper, creating a clean edge for the mousse. 

Pour the mousse over the cake, smoothing the top with an offset spatula. Let it chill in the refrigerator for no less than 6 hours for the best results.

For the ganache, combine the dark chocolate and heavy cream in a heatproof bowl. 

Microwave for about 60 seconds, then let it sit for 10 minutes to melt. Stir until smooth. If the ganache is too thick, warm it for a few seconds; if it's too thin, let it cool a bit to thicken.

Once the mousse has set, remove the springform pan and peel away the acetate. 

Pour the slightly warm ganache over the mousse, smoothing it out to the edges. Place in the refrigerator for 30 minutes to let the ganache firm up.

Slice with a sharp knife, wiping the blade between cuts for clean edges, and enjoy your homemade chocolate mousse cake!