Chocolate mixture: In a small bowl, add the chopped chocolate, espresso powder, and vanilla extract.
Heat cream: Heat half (200g) of the whipping cream in a saucepan on medium heat or in the microwave until it just begins to simmer. Pour the hot cream over the chopped chocolate and let it sit undisturbed for 2 minutes. Then, stir until all the chocolate has melted. Set aside.
Whip cream: To a medium mixing bowl, add the remaining 200g whipping cream. Whisk until it becomes soft peaks.
Fold together: Add the melted chocolate mixture to the whipped cream and fold together with a spatula until no streaks remain.
Fill serving glasses: Transfer the chocolate mousse mixture to your serving glasses or containers of your choice. I overfilled coupe glasses and used a bench scraper to scrape the excess off the top for a perfectly flat surface.
Chill: Chill the mousse in the fridge for at least 4 hours or overnight until completely set.
Toppings
Whip cream: In a small mixing bowl, add the whipping cream, powdered sugar, and vanilla extract. Whisk together until it becomes firm peaks.
Garnish: Add a dollop of whipped cream on top of each chocolate mousse cup. Place half a strawberry on top of the whipped cream. Use a microplane add some chocolate shavings on top.
Serve: When serving, serve straight from the fridge for a fudgier texture or let the mousse cups come back to room temperature for about 15 minutes for a creamier texture.