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

In a large mixing bowl, whisk together flour, cocoa powder, espresso powder, baking powder, baking soda, and sea salt.

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

Sift the dry mixture into the wet ingredients and gently fold in, then add boiling water and whisk until just mixed.

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

For the mousse, whisk heavy cream, eggs, sugar, cocoa powder, espresso powder, and salt in a saucepan off the heat until mixed, then heat until it reaches 170ºF (76ºC).

Remove from heat, add chocolate and vanilla, and whisk until smooth. Cool for at least 30 minutes.

Whip additional cream to soft peaks and fold into cooled chocolate mixture in batches.

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

To make ganache, heat cream until simmering and pour over chopped chocolate. Stir until smooth and pour over chilled mousse.

Let ganache set for 15 minutes before serving.