In a small saucepan, whisk together the granulated sugar and eggs until well blended. Cook over low heat, whisking constantly, until the mixture reaches 160 degrees F and coats the back of a metal spoon. Remove from the heat. Add the chocolate and vanilla and stir until smooth. Set aside to cool for approximately 5 minutes.

In a stand mixer fitted with the paddle attachment, cream the butter on medium-high speed until light and fluffy. Add the cooled chocolate mixture and beat on high speed until light and fluffy, about 5 minutes.

In another large bowl, beat the cream on medium-high speed until it begins to thicken, 3 to 4 minutes. Add the powdered sugar and beat on low speed, then gradually return to medium-high until stiff peaks form. Fold into the chocolate mixture.

Pour into the prebaked pie crust or tart shell. Refrigerate until well chilled, at least 4 hours or up to overnight.

Garnish with whipped cream and shaved chocolate, if desired.

Store, covered, in the refrigerator for 4 to 5 days.

Preheat the oven to 350 degrees F.

Pulse the wafers on low in a food processor until they are the consistency of sand. Add the sugar and melted butter and mix well.

Press the mixture into the bottom and up the sides of a 9-inch pie pan. Bake 8 minutes.

Let cool 20 minutes before filling.