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

In a separate bowl, combine white granulated sugar, neutral oil, full-fat sour cream, whole eggs, and pure vanilla extract. Whisk until smooth.

Sift the dry ingredient mixture into the wet ingredients, stirring until just combined. Gradually add boiling water while mixing.

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

Cool the cake on a wire rack for about 15 minutes, then remove from the pan and let it cool completely.

Whisk together heavy cream, egg yolk, sugar, Dutch cocoa powder, optional espresso powder, and salt in a saucepan over low heat until reaching 170ºF (76ºC).

Remove from heat and stir in dark chocolate and pure vanilla extract until fully melted and smooth. Refrigerate until thickened.

In a separate bowl, whip the cold heavy cream until firm peaks form, then fold into the chocolate mixture until no streaks remain.

Place the cooled cake back in the springform pan. Pour the mousse over evenly and smooth the top. Refrigerate for at least 8 hours or overnight.

For the ganache, heat heavy cream until it simmers, then pour over dark chocolate, stirring until smooth. Allow to cool slightly before pouring over the mousse.