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

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

In another bowl, combine granulated sugar, neutral oil, sour cream, egg, and pure vanilla extract. Whisk until smooth and creamy.

Gently fold the dry mixture into the wet ingredients and then carefully add boiling water, mixing until smooth.

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

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

For the mousse, whisk together heavy cream, egg, egg yolk, granulated sugar, cocoa powder, espresso, and salt in a saucepan over medium-low heat until it reaches 170ºF (76ºC).

Remove from heat, stir in dark chocolate and vanilla until melted and smooth, then chill in the refrigerator until thickened.

Whip cold heavy cream in a separate bowl until firm peaks form and fold into the cooled chocolate mixture.

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

For the ganache, heat heavy cream until bubbling, pour over chopped dark chocolate, stir until smooth.

After the mousse has set, pour ganache over the cake, allowing it to drip down the sides before slicing.