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

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.

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.

Cover the pastry with parchment paper and fill it with pie weights or dried beans. Bake at 350°F for 15–17 minutes, then carefully remove the weights and parchment. If the base is still pale or damp, continue baking without the weights until it is lightly golden, dry, and fully baked.

Allow the fully baked pastry crust to cool completely in the pan before adding the filling.

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.

Whisk the eggs and granulated sugar together in a medium saucepan. Cook over low heat, stirring constantly, until the sugar dissolves and the mixture reaches 160°F (71°C) on a thermometer. About 5 minutes is a guide; use the temperature as the deciding check.

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.

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.

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.

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.

Measure 1 1/2 cups of the already-whipped cream and gently fold it into the chocolate mixture until evenly incorporated. Keep the remaining whipped cream refrigerated for the topping.

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

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

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