In a mixing bowl, combine the chocolate cookie crumbs and melted butter. Stir until the mixture resembles wet sand.

Press the cookie mixture firmly into the bottom and up the sides of a 9-inch pie dish to form the crust. Refrigerate for at least 30 minutes to set.

Meanwhile, melt the chocolate chips using a double boiler or in a microwave-safe bowl in 30-second intervals, stirring until smooth. Allow to cool slightly.

In a separate bowl, beat the egg yolks, remaining sugar, and vanilla extract until thick and pale in color.

Gradually add the melted chocolate to the egg yolk mixture, mixing until fully combined.

In another bowl, beat the heavy whipping cream until stiff peaks form. Reserve about ½ cup for topping.

Gently fold the chocolate mixture into the whipped cream until completely incorporated.

In a clean bowl, beat the egg whites with a pinch of salt until soft peaks form. Carefully fold the beaten egg whites into the chocolate mousse mixture, being cautious not to deflate the volume.

Pour the mousse filling into the chilled crust and spread it evenly. Cover with plastic wrap and refrigerate for at least 3 hours or until set.

Once set, remove from the refrigerator, and garnish with reserved whipped cream if desired. Slice and serve.