Prepare the oven and pan. Preheat the oven to 350°F. Lightly coat a half sheet pan with nonstick cooking spray and set it aside.

Shape the pie crust. Place the two softened pie crusts on a lightly floured work surface, stacking one over the other. Roll them together into a rectangle slightly larger than the sheet pan.

Fit the crust into the pan. Carefully transfer the rolled dough to the prepared pan. Press it gently across the bottom and into the corners, trim away any excess dough, then crimp or shape the edges as desired.

Blind bake the crust. Cover the pastry with parchment paper and fill it with pie weights or dried beans. Bake for 15-17 minutes, or until the crust is lightly golden and cooked through.

Cool the crust. Remove the pie weights and parchment, then allow the baked crust to cool completely before adding the filling.

Melt the chocolate. Place the bittersweet chocolate in a microwave-safe bowl. Microwave in 30-second intervals, stirring after each interval, until almost completely melted. Continue stirring until smooth, then set aside.

Cook the egg mixture. Add the eggs and granulated sugar to a medium saucepan and whisk until combined. Cook over low heat while stirring constantly for about 5 minutes, or until the sugar dissolves and the mixture thickens enough to lightly coat the back of a spoon. For food-safety assurance, the egg mixture should reach at least 160°F.

Add the chocolate. Remove the saucepan from the heat. Stir in the melted chocolate and vanilla until the mixture is completely smooth, then allow it to cool until warm, stirring occasionally.

Whip the butter. Place the softened butter in a large mixing bowl and beat with an electric mixer for 2-3 minutes, or until noticeably lighter and fluffy.

Finish the chocolate mixture. Slowly pour the cooled chocolate mixture into the whipped butter while mixing. Once incorporated, increase the mixer speed and beat for about 5 minutes, until the filling becomes lighter and fluffy.

Prepare the whipped cream. In a separate mixing bowl, beat the heavy whipping cream until it begins to thicken. Add the powdered sugar and continue beating until stiff peaks form.

Lighten the filling. Gently fold 1 1/2 cups of the prepared whipped cream into the chocolate mixture until evenly incorporated. Refrigerate or set aside the remaining whipped cream for the topping.

Fill the crust. Spoon the French silk filling into the completely cooled pastry crust and spread it into an even layer.

Chill. Refrigerate the slab pie for 1-2 hours, or until the chocolate filling is properly set.

Decorate and serve. Spread or pipe the reserved whipped cream over the chilled chocolate filling. Finish with chocolate curls or shavings, slice into squares, and serve cold.