1. Preheat Oven: Preheat your oven to 350°F (175°C).

2. Make Crust: In a mixing bowl, combine graham cracker crumbs, melted butter, and sugar. Mix well until combined.

3. Press into Pan: Firmly press the mixture into the bottom and up the sides of a 9-inch pie pan. Bake for 10 minutes, then allow it to cool completely.

4. Melt Chocolate: In a heatproof bowl, melt the chopped chocolate and butter together over a pot of simmering water or in the microwave in 30-second increments, stirring until smooth.

5. Whisk Eggs: In another bowl, whisk together the eggs and sugar until pale and fluffy.

6. Combine Mixtures: Gently fold the melted chocolate into the egg mixture along with the vanilla extract until fully incorporated.

7. Fold in Cream: Carefully fold the whipped cream into the chocolate mixture until no white streaks remain.

8. Pour into Crust: Pour the mousse filling into the cooled crust, smoothing the top as needed.

9. Chill: Refrigerate the pie for at least 1 hour or until set.

10. Garnish and Serve: Before serving, garnish the pie with chocolate shavings and fresh berries if desired.