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 all-purpose flour, Dutch cocoa powder, espresso powder, baking powder, baking soda, and sea salt.

In another bowl, mix the granulated sugar, neutral oil, full-fat sour cream, whole egg, and vanilla extract until smooth.

Sift the dry ingredients into the wet mixture, then gradually stir in the boiling water until just combined.

Pour the cake batter into the prepared springform pan and bake for 24-28 minutes.

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

In a saucepan, whisk together heavy cream, whole egg, egg yolk, granulated sugar, cocoa powder, espresso powder, and a pinch of salt off heat.

Heat the saucepan over low heat, whisking until the mixture reaches 170ºF (76ºC). Remove from heat and stir in dark chocolate and vanilla extract until smooth.

Allow the chocolate mixture to cool slightly, then whip the remaining heavy cream to firm peaks and fold it into the chocolate mixture.

Place the cooled chocolate cake back into the springform pan, spread the mousse evenly on top, and refrigerate for at least 8 hours.

For the ganache, heat heavy cream in a small saucepan until simmering and pour it over chopped dark chocolate, stirring until smooth.

Allow the ganache to set slightly before slicing the mousse cake and serving, garnished with chocolate shavings or fresh berries if desired.