Preheat your oven to 350°F (175°C). In a medium bowl, combine chocolate cookie crumbs, melted butter, and sugar until well incorporated.

Press the mixture into a 9-inch tart pan and bake for 10 minutes. Allow to cool completely.

Melt dark chocolate in a microwave-safe bowl until smooth, then let it cool slightly.

Whip 1 cup of heavy whipping cream until soft peaks form, then fold it into the cooled chocolate.

In another bowl, beat egg whites with a pinch of salt until medium peaks form. Gradually add sugar until stiff peaks form.

Fold the egg white mixture into the chocolate mixture, being careful not to deflate it.

Add peppermint extract to the mousse and fold gently to incorporate.

Spoon the mousse into the cooled tart shell and smooth the top with a spatula.

Cover the tart and refrigerate for at least 4 hours to set.

Whip the remaining 1 cup of heavy whipping cream to stiff peaks, then spread it over the tart.

Garnish with crushed peppermint candies and shaved chocolate before serving.

Slice the tart and serve chilled.