Preheat your oven to 350ºF (180ºC) and grease a 9-inch springform pan. In a large mixing bowl, combine the dry ingredients: flour, cocoa powder, espresso powder, baking powder, baking soda, and salt.

In another bowl, whisk together the wet ingredients: sugar, oil, sour cream, egg, and vanilla. Sift the dry mixture into the wet, mixing gently until just combined, then stir in the boiling water.

Pour the cake batter into the prepared springform pan and bake for 24-28 minutes, until a toothpick comes out clean. Allow to cool in the pan for 15 minutes before transferring to a wire rack.

In a medium saucepan, whisk together heavy cream, egg yolk, sugar, cocoa, espresso, and salt over low heat, stirring until it reaches 170ºF (76ºC). Remove from heat and stir in dark chocolate and vanilla.

Once the cake is cooled, assemble it in the springform pan. Spread the mousse over the cake, seal with plastic wrap, and chill for at least 8 hours.

To make ganache, heat heavy cream until simmering. Pour over chopped dark chocolate, letting it sit, then stir until smooth.

Remove the cake from the pan, slice, and serve chilled, optionally topped with cocoa powder or fresh berries.