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

In a large bowl, combine the dry ingredients: flour, cocoa powder, espresso powder, baking powder, baking soda, and salt. Whisk until well blended.

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

Sift the dry ingredients into the wet mixture and fold gently to combine. Slowly stir in the boiling water until blended.

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

In a saucepan, whisk together the cream, egg, egg yolk, sugar, cocoa powder, and salt. Heat gently until it reaches 170ºF (76ºC).

Remove from heat and stir in the chopped dark chocolate and vanilla extract until smooth. Let cool at room temperature.

Whip cream until firm peaks form, then fold gently into the cooled chocolate mixture.

Reinsert the cooled cake into the springform pan and pour the mousse over it. Smooth the top and refrigerate for at least 8 hours.

Heat heavy cream until almost simmering, pour over chopped dark chocolate, and stir until smooth.

Remove cake from the pan, pour ganache over the mousse layer, and chill briefly before serving.