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

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

In a large mixing bowl, combine granulated sugar, neutral oil, full-fat sour cream, eggs, and pure vanilla extract. Mix until smooth and creamy.

Sift the dry ingredients into the bowl with the wet mixture, stirring until nearly incorporated. Gradually add the boiling water, mixing until just combined.

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

Let the cake cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.

In a saucepan over low heat, whisk together heavy cream, a whole egg, egg yolk, granulated sugar, cocoa powder, espresso powder, and salt until the mixture reaches 170ºF (76ºC).

Mix in the dark chocolate and vanilla extract until fully melted and smooth, then chill in the refrigerator for about 1-1.5 hours.

Whip cold heavy cream in a mixing bowl until stiff peaks form, then gently fold into the chilled chocolate mixture.

Once the cake is completely cool, pour the chocolate mousse over the top and chill the assembled cake for at least 8 hours.

Heat additional heavy cream until just steaming, then pour it over chopped dark chocolate and stir until smooth for the ganache.

Pour the silky ganache over the chilled mousse layer, slice the cake carefully, and serve with fresh berries or whipped cream.