TO MAKE THE CRUST

Preheat oven to 350 degrees F.

Place the Biscoff cookies into a food processor. Pulse until cookies are crumbly.

Pour the crumbs into a bowl and stir in the melted butter.

Pour the buttery crumbs into a pie pan. Press the crumbs into the bottom and sides of the pan using your hands.

Bake the crust for 8 minutes. Remove from oven and set aside to cool.

TO MAKE THE CHOCOLATE MOUSSE

Put the chocolate chips and butter in a heatproof bowl and place over a large pot containing about 1-inch of simmering water (or use a double boiler).

Stir until chocolate and butter is melted and the mixture is smooth.

Remove the bowl from the heat and allow to cool, about 5 minutes.

Whisk in the egg yolks, 1 at a time, whisking until smooth. Set aside.

In another large mixing bowl, beat the egg whites until foamy. Add the cream of tartar and beat until soft peaks form. Slowly add 1/4 cup sugar and continue beating until peaks become firm. Set aside.

In another bowl, beat the heavy cream until it begins to thicken. Add the remaining 1 tablespoon sugar and the vanilla and continue beating until soft peaks form.

Gently fold the egg white mixture into the chocolate mixture. Next, fold in the whipped cream.

Spoon the chocolate mousse into the pie crust. Smooth out the top with a spatula. Cover and refrigerate for at least 6 hours or preferably overnight.

Top with whipped cream, if desired.

TO MAKE WHIPPED CREAM TOPPING

Pour the heavy cream into a mixing bowl. Add the sugar and whisk until the cream reaches stiff peaks.