Preheat your oven to 350°F (175°C). In a mixing bowl, combine the finely ground chocolate graham crackers, melted butter, and sugar until well mixed. Press this mixture firmly into the bottom and up the sides of a tart pan. Bake for 10 minutes, then cool on a wire rack.

In a small bowl, sprinkle the unflavored gelatin over the cold water and let it bloom for about 5 minutes. Heat gently until dissolved.

Melt the white chocolate chips in a microwave-safe bowl in 30-second intervals, stirring until smooth. Heat the milk until steaming and combine with the melted chocolate and dissolved gelatin.

Whip the heavy whipping cream with powdered sugar until medium-stiff peaks form.

Fold one-third of the whipped cream into the chocolate mixture to lighten it, then fold in the remaining whipped cream until no streaks remain.

Spoon the mousse into the cooled crust and refrigerate for several hours or overnight.

To make the glaze, heat the heavy whipping cream until boiling, remove from heat and stir in chocolate until smooth. Mix in corn syrup and warm water.

Pour the glaze over the tart and let set for an hour.