Prepare the Crust

1. Mix the Ingredients: In a bowl, combine the graham cracker crumbs, melted butter, and sugar until the mixture resembles wet sand.
2. Press Into Molds: Divide the mixture evenly among six small ramekins or dessert glasses. Press down firmly to create a compact layer.
3. Chill: Place the ramekins in the refrigerator to set while you prepare the mousse.

Make the Chocolate Mousse

1. Melt the Chocolate: In a heatproof bowl, melt the dark chocolate and butter over a double boiler or in the microwave in 30-second intervals. Stir until smooth. Add espresso, if using, and let cool slightly.
2. Whip the Cream: In a separate bowl, beat the heavy cream, powdered sugar, and vanilla extract until soft peaks form.
3. Fold Together: Gently fold the whipped cream into the cooled chocolate mixture in batches, ensuring a light and airy texture.
4. Assemble the Desserts: Spoon or pipe the mousse over the prepared crusts, smoothing the tops.

Chill and Serve

1. Refrigerate: Let the desserts chill for at least 1 hour, allowing the mousse to set.
2. Garnish: Before serving, add a dollop of whipped cream, a sprinkle of shaved chocolate, or fresh berries for a touch of elegance.