Preheat your oven to 350ºF (180ºC) and prepare a 9-inch springform pan by lining the bottom with parchment paper and greasing the sides.

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

In another bowl, whisk together the granulated sugar, neutral oil, full-fat sour cream, whole large eggs, and pure vanilla extract until smooth and creamy.

Gently fold the dry ingredients into the wet mixture, then add the boiling water, whisking until just combined.

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

In a saucepan off heat, whisk together the heavy cream, egg yolk, granulated sugar, Dutch cocoa powder, fine espresso powder, and fine sea salt. Cook over low heat until it reaches 170ºF (76ºC).

Remove from heat, stir in dark chocolate until smooth and glossy. Cool at room temperature for 10 minutes, then refrigerate for 1 to 1.5 hours.

Whip the cold heavy cream until firm peaks form.

Fold the whipped cream into the cooled chocolate mixture until no white streaks remain.

Assemble the cake by placing the cooled chocolate cake at the bottom of the springform pan and pouring the mousse over it. Chill for at least 8 hours.

Prepare ganache by heating heavy cream until simmering, pour over chopped dark chocolate, stir until smooth.

Pour ganache over the chilled cake, refrigerate for 30 minutes, then slice and serve.