Preheat your oven to 350ºF (180ºC) and grease a 9-inch springform pan with butter or oil, lining the bottom with parchment paper.

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

In a large mixing bowl, whisk together granulated sugar, neutral oil, full-fat sour cream, whole eggs, and pure vanilla extract until smooth.

Sift the dry ingredients into the wet mixture, gently folding until nearly combined, then add boiling water and mix until just combined.

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

In a saucepan, combine heavy cream, egg, egg yolk, sugar, cocoa powder, fine espresso powder, and salt, heating until 170ºF (76ºC). Stir in dark chocolate and vanilla until smooth.

Once the cake has cooled, pour the chilled mousse evenly over the top and chill in the refrigerator for at least 8 hours.

Heat heavy cream in a saucepan until simmering, pour over chopped dark chocolate, let rest, and stir until smooth to make the ganache.

Unmold the chilled cake, pour ganache over the top, chill briefly, then slice and serve your Dark Chocolate Mousse Cake.