Preheat your oven to 350°F (175°C) to prepare for baking the tart crust.

In a medium bowl, combine the chocolate cookie crumbs, melted unsalted butter, and granulated sugar. Mix thoroughly until resembling wet sand.

Press the cookie mixture evenly into the bottom and up the sides of your greased tart pan. Bake for 10 minutes and let cool completely.

In a microwave-safe bowl, melt the chopped dark chocolate in 30-second intervals until smooth. Allow to cool slightly.

In a separate bowl, whip the heavy cream on medium speed until soft peaks form.

Fold the whipped cream into the melted chocolate until there are no streaks remaining.

Beat the egg whites with a pinch of salt until medium peaks form. Gradually add granulated sugar until stiff peaks form.

Gently fold the beaten egg whites into the chocolate mixture until fully incorporated.

Drizzle peppermint extract into the mousse and gently fold to combine.

Spoon the mousse mixture into the cooled tart shell, smoothing the top.

Chill in the refrigerator for at least 4 hours until the mousse is set.

Before serving, whip remaining heavy cream and spread it generously over the tart. Garnish with crushed peppermint candies and chocolate shavings.