Preheat your oven to 350ºF (180ºC) and grease a 9-inch springform pan.

In a small mixing bowl, combine dry ingredients: flour, cocoa powder, espresso powder, baking powder, baking soda, and salt.

In a larger bowl, blend wet ingredients: sugar, oil, sour cream, egg, and vanilla.

Sift dry mix into wet mix, add boiling water, and whisk until combined.

Pour batter into pan and bake for 24 to 28 minutes.

Cool the cake on a wire rack for 15 minutes before removing from the pan.

In a saucepan, whisk together heavy cream, whole egg, yolk, sugar, cocoa powder, espresso powder, and salt.

Heat mixture while whisking until it reaches 170ºF (76ºC). Don't let it boil.

Remove from heat and whisk in dark chocolate and vanilla until smooth, then chill for 1-1.5 hours.

Prepare the springform pan with parchment; level the cooled cake if necessary.

Wrap cake sides in acetate or parchment to prevent mousse from sticking.

Whip the cold heavy cream until firm peaks form.

Fold chocolate mixture into whipped cream until no white streaks remain.

Spread mousse over the cake in the springform pan and chill for at least 8 hours.

For ganache, heat heavy cream until simmering, pour over chopped dark chocolate, stir until smooth.

Remove cake from fridge, unmold it, and pour the ganache over the top.

Chill for an additional 10-15 minutes before serving.