Preheat your oven to 350ºF (180ºC) and prepare a 9-inch springform pan by greasing it.

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

In a separate bowl, whisk together the white granulated sugar, neutral oil, full-fat sour cream, large egg, and pure vanilla extract until smooth.

Sift the dry mixture into the wet mixture, then add boiling water, stirring until fully incorporated.

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

Remove and let the cake cool in the pan for 15 minutes before transferring to a wire rack.

In a medium saucepan, combine heavy cream, egg, yolk, sugar, cocoa powder, espresso powder, and a pinch of salt. Cook over low heat until the mixture reaches 170ºF (76ºC).

Remove from heat, add dark chocolate and vanilla extract, whisk until smooth, then let it cool.

Whip the additional heavy cream to stiff peaks and gently fold it into the cooled chocolate mixture.

Place the cooled chocolate cake back in the springform pan, pour in the mousse, and chill for at least 8 hours.

Prepare the ganache by combining dark chocolate and warm heavy cream until smooth.

Once the ganache has set, remove the cake from the springform pan and slice to serve.