Preheat your oven to 350°F (175°C).

In a medium bowl, combine chocolate wafer cookie crumbs, melted butter, and sugar until it resembles wet sand.

Press this crumb mixture evenly into a 9-inch pie pan to form the crust.

Bake the crust for 10 minutes, then let it cool completely.

In a small saucepan, combine heavy cream and espresso powder. Warm over medium heat until hot but not boiling.

Remove from heat and add chopped semisweet chocolate, vanilla extract, and salt. Stir until smooth.

In a separate bowl, whisk egg yolks. Gradually add a small amount of the warm chocolate mixture to the yolks, whisking constantly to temper the eggs.

Return the tempered egg mixture to the saucepan and cook over low heat, stirring until it thickens slightly (about 5 minutes); do not boil.

Stir in coffee liqueur if using, then pour the chocolate filling into the cooled crust.

Refrigerate for at least 4 hours or until set.

Before serving, top with whipped cream and chocolate shavings.