Preheat your oven to 350 degrees Fahrenheit. In a medium bowl, combine the chocolate sandwich cookie crumbs with the melted butter and stir until the mixture resembles wet sand. Press the mixture firmly into the bottom and up the sides of a 9 inch tart pan with a removable bottom. Bake for 10 minutes to set the crust, then remove from the oven and let it cool completely on a wire rack.

In a small saucepan, whisk together the water and the agar-agar powder. Let it sit for about 5 minutes to bloom. Place the saucepan over low heat and stir constantly until the powder is fully dissolved and the liquid begins to simmer slightly. Remove from the heat and set aside to cool for a minute, but do not let it set.

Place the chopped dark chocolate and half a cup of the heavy cream in a microwave-safe bowl. Heat in 30-second intervals, stirring in between, until the chocolate is completely smooth and melted. Stir in the alcohol-free peppermint extract, alcohol-free vanilla, salt, and the prepared agar-agar mixture. Mix well until everything is glossy and combined, then let it cool to room temperature.

In a large chilled bowl, beat the remaining one and a half cups of heavy whipping cream until stiff peaks form. Be careful not to over-beat, or the cream will turn into butter. Gently fold about one-third of the whipped cream into the cooled chocolate mixture to lighten it up. Then, fold in the remaining whipped cream using a rubber spatula, moving in a slow circular motion until no white streaks remain.

Pour the chocolate mousse filling into the cooled tart shell and smooth the top with a spatula. Place the tart in the refrigerator for at least 4 hours, or preferably overnight, to allow the mousse to set firmly. Just before serving, sprinkle the crushed candy canes around the edges of the tart for a beautiful holiday finish. Carefully remove the tart from the outer ring of the pan and slice into wedges.