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

In a medium bowl, whisk together the all-purpose flour, Dutch cocoa powder, fine espresso powder (if using), baking powder, baking soda, and fine sea salt until uniformly combined.

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

Gradually sift the dry ingredients into the wet mixture, folding gently. Pour in the boiling water, stirring briskly until just combined.

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

Combine heavy cream, egg yolk, granulated sugar, Dutch cocoa powder, optional espresso powder, and a pinch of salt in a saucepan off heat. Stir gently to mix.

Place the saucepan over low heat and cook while stirring until it reaches 170ºF (76ºC). Add dark chocolate and vanilla extract until melted and combined.

Transfer to a bowl, cover with plastic wrap, and refrigerate until thickened, about 1 to 2 hours.

Whip the additional cold heavy cream in a clean bowl until firm peaks form.

Fold in the whipped cream into the cooled chocolate mixture gently.

Place the cooled chocolate cake back into the springform pan, wrap it with a cake collar, and spread the mousse evenly on top.

Refrigerate for at least 8 hours to set.

Heat heavy cream until it begins to simmer, then pour over chopped dark chocolate in a bowl and stir until melted and smooth.

Once the cake has fully chilled, unmold it and pour the ganache over the top, allowing it to set before slicing.

Serve with fresh berries or whipped cream for an extra touch.