Preheat your oven to 350°F (180°C). Grease a 9-inch springform pan with butter and line the bottom 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.

In another bowl, whisk together white granulated sugar, neutral oil, sour cream, egg, and vanilla extract until smooth.

Sift the dry ingredients into the wet ingredients and mix until just combined.

Carefully incorporate boiling water into the batter until fully mixed.

Pour the batter into the prepared springform pan and bake for 24-28 minutes.

Remove the cake from the oven and cool in the pan for 10 minutes before transferring to a wire rack to cool completely.

Prepare the chocolate mousse by whisking heavy cream, egg yolk, sugar, cocoa powder, espresso powder, and salt in a saucepan over low heat until thickened.

Remove from heat, add chopped dark chocolate and vanilla extract, and stir until smooth.

In a separate bowl, whip cold heavy cream until firm peaks form and fold into the cooled chocolate mixture.

Return the cooled cake to the springform pan, spread mousse evenly on top, and refrigerate for at least 8 hours.

For the ganache, heat heavy cream until simmering and pour over chopped dark chocolate, stirring until smooth. Once the mousse is set, pour ganache over.