Preheat your oven to 350°F (180°C) and prepare a 9-inch springform pan by lining the bottom with parchment paper.

In a large mixing bowl, whisk together the all-purpose flour, Dutch cocoa powder, fine espresso powder, baking powder, baking soda, and fine sea salt.

In a separate bowl, combine the white granulated sugar, neutral oil, full-fat sour cream, large eggs, and pure vanilla extract.

Carefully add the dry ingredients into the wet mixture, folding gently to avoid over-mixing. Then pour in the boiling water and mix lightly until uniform.

Pour the batter into the prepared springform pan and bake for 24–28 minutes, or until a toothpick inserted comes out clean.

In a saucepan off heat, whisk together the heavy cream, egg, egg yolk, sugar, Dutch cocoa powder, fine espresso powder, and a pinch of salt. Place on low heat until thickened to 170°F (76°C).

Remove from heat and whisk in the dark chocolate until melted and smooth. Stir in pure vanilla extract.

Chill the mousse mixture in the refrigerator until it thickens, about 30-40 minutes.

Pour cold heavy cream into a mixing bowl and whip until soft peaks form. Fold the whipped cream gently into the chilled mousse.

Spread the chocolate mousse evenly over the top of the cooled cake in the springform pan. Cover and chill for at least 8 hours.

To create the ganache, heat heavy cream until it simmers and pour over chopped dark chocolate. Stir until smooth and glossy.

Drizzle the ganache over the chilled cake and serve.