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

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

In a separate bowl, blend white granulated sugar and neutral oil until creamy. Add sour cream, egg, and vanilla, mixing thoroughly until smooth.

Gradually add the dry ingredients to the wet mixture, whisking gently. Slowly fold in boiling water until the batter is glossy and well combined.

Pour the batter into the prepared pan and bake for 24-28 minutes until a toothpick comes out clean. Allow it to cool on a wire rack.

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

Remove from heat and stir in dark chocolate until melted and smooth. Allow to cool at room temperature for about an hour.

In a separate bowl, beat additional heavy cream until stiff peaks form. Gently fold the whipped cream into the cooled chocolate mixture.

Once the cake has cooled, add the mousse on top and refrigerate for at least 8 hours, ideally overnight.

To prepare the ganache, heat cream until it simmers, pour over chopped chocolate, and stir until smooth. Cool slightly before pouring over the mousse layer.