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

Whisk together the dry ingredients: all-purpose flour, cocoa powder, espresso powder, baking powder, baking soda, and salt.

In a large bowl, mix sugar, oil, sour cream, egg, and vanilla extract until smooth.

Sift dry ingredients into wet ingredients and fold gently. Add boiling water gradually.

Pour batter into springform pan and bake for 24–28 minutes or until a toothpick comes out clean.

Let the cake cool in the pan for 15 minutes, then remove sides and cool completely on a wire rack.

In a medium saucepan, whisk heavy cream, egg, egg yolk, sugar, cocoa powder, espresso powder, and salt. Heat until it reaches 170ºF (76ºC).

Remove from heat, add dark chocolate and vanilla extract, and whisk until smooth. Let cool until thickened.

Whip additional heavy cream until firm peaks form and gently fold into cooled chocolate mixture.

Place cooled cake back in springform pan, wrap the sides with acetate, and pour the mousse over the top. Chill for at least 8 hours.

To make ganache, heat heavy cream and pour over chopped dark chocolate, stirring until smooth.

Once set, pour ganache over the mousse cake, spreading gently to the edges.