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

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

In a larger mixing bowl, combine granulated sugar, neutral oil, full-fat sour cream, the whole egg, and vanilla extract. Whisk until smooth and creamy.

Sift the dry mixture into the wet ingredients, gently whisking until just combined. Slowly add boiling water and stir until the batter is smooth.

Pour the batter into the prepared pan, smoothing the top. Bake for 24-28 minutes or until a toothpick comes out clean.

Let the cake cool in the pan for 15 minutes, then transfer to a wire rack to cool completely.

In a saucepan, whisk heavy cream, whole egg, egg yolk, granulated sugar, cocoa powder, espresso powder, and salt. Cook on low heat until the mixture reaches 170ºF.

Remove from heat, stir in chopped dark chocolate and vanilla extract until melted and smooth. Chill in the refrigerator for 1–1.5 hours.

Whip remaining heavy cream until firm peaks form, then fold it into the chocolate mixture until well-blended but light.

Wrap a cake collar around the cooled cake, pour mousse over the top, and refrigerate for at least 8 hours.

For the ganache, heat heavy cream until simmering, then pour over chopped dark chocolate, stirring until smooth.

Unmold the cake, drizzle with ganache, slice with a hot knife, and serve garnished as desired.