Preheat your oven to 350ºF (180ºC). Grease a 9-inch (23cm) springform pan and line the bottom with parchment paper.

In a small mixing bowl, whisk together the flour, cocoa, espresso powder (if using), baking powder, baking soda, and salt.

In a large mixing bowl, whisk together the sugar, oil, sour cream, egg, and vanilla until smooth.

Sift the dry ingredients into the wet ingredients and gently whisk. Slowly add boiling water and whisk until combined.

Pour the batter into the prepared pan and bake for 24 to 28 minutes. Cool in the pan for 15 minutes, then remove to cool completely.

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

Place over low heat and whisk until it reaches 170ºF. Remove from heat, add chocolate and vanilla, and stir until smooth.

Allow the mixture to cool for about an hour until it thickens and becomes spreadable.

Line the springform pan bottom with parchment. Level the cake if necessary, and wrap sides with a cake collar.

Whip heavy cream until firm peaks form. Gently fold the mousse into the whipped cream until no white streaks remain.

Spread the mousse over the cooled cake layer, smooth it out, cover with plastic wrap, and chill for at least 8 hours.

For ganache, place chocolate in a bowl. In a saucepan, bring cream to a simmer, pour over chocolate, let sit, and stir until smooth.

Carefully remove the chilled cake from the fridge. Pour ganache over the mousse layer, spreading it gently. Chill for 10-15 minutes before serving.