Preheat your oven to 350°F (175°C). In a mixing bowl, combine crushed chocolate graham crackers, melted butter, and sugar until well mixed. Press this mixture tightly into the bottom and up the sides of a tart pan. Bake for 10 minutes until the crust sets and is lightly golden. Allow it to cool completely before adding the mousse.

In a small bowl, sprinkle unflavored gelatin over cold water and let it bloom for 5 minutes. Once it's set, microwave the gelatin carefully until just melted—do not boil.

In another bowl, melt white chocolate, then whisk in milk until smooth. Stir in the gelatin mixture and peppermint oil after slightly cooling.

In a separate chilled mixing bowl, beat heavy whipping cream with powdered sugar until medium-stiff peaks form. Gently fold the whipped cream into the chocolate mixture in three additions, ensuring it's well combined and airy.

Spoon the peppermint mousse into the cooled tart crust, smoothing out the top. Cover the tart with plastic wrap and refrigerate for at least 4 hours or preferably overnight.

In a saucepan, heat heavy cream over medium heat until it reaches a simmer. Remove from heat, then add chopped dark chocolate and stir until completely melted and smooth. Mix in light corn syrup and very warm water until the glaze is glossy.

Carefully pour the cooled dark chocolate glaze over the chilled peppermint mousse, ensuring it coats evenly. Allow the glaze to set at room temperature for about 1 hour before slicing.