Preheat your oven to 350ºF (180ºC) and prepare a greased 9-inch springform pan lined with parchment paper.

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

In a separate bowl, whisk together granulated sugar, neutral oil, sour cream, whole egg, and vanilla extract until smooth.

Sift the dry mixture into the wet mixture and combine gently. Gradually add boiling water, mixing until smooth.

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

Once baked, cool the cake completely in the pan on a wire rack.

In a saucepan, whisk together heavy cream, sugar, cocoa powder, espresso, and a pinch of salt over low heat until it reaches 170ºF (76ºC).

Remove from heat and stir in dark chocolate and vanilla extract until smooth, then let cool for 15-20 minutes.

Whip remaining heavy cream until stiff peaks form and fold gently into the cooled chocolate mixture.

Spread the mousse evenly over the cooled chocolate cake and refrigerate for at least 8 hours to set.

To make ganache, heat heavy cream until simmering, pour over chopped dark chocolate, and stir until glossy.

Once the mousse has set, pour the ganache over the cake and serve with fresh berries or whipped cream.