Preheat your oven to 350°F (175°C). Combine ground chocolate graham crackers with melted butter and sugar in a mixing bowl. Press firmly into the bottom and sides of a tart pan. Bake for 10 minutes. Allow to cool completely.

Bloom the unflavored gelatin in cold water for about 5 minutes. Dissolve the gelatin in the microwave for 10-15 seconds until melted. Melt white chocolate with milk in a saucepan over low heat, stirring until smooth. Whisk in gelatin and peppermint oil.

Whip heavy cream and powdered sugar until medium-stiff peaks form, about 3-5 minutes. Fold a dollop into chocolate mixture to lighten it, then fold in the remaining whipped cream until no white streaks remain.

Spoon the peppermint mousse mixture into the cooled crust and smooth the top with a spatula. Cover with plastic wrap and refrigerate for at least 4 hours, preferably overnight.

Heat heavy cream in a saucepan over medium heat until boiling. Remove from heat, add bittersweet chocolate, stirring until smooth. Stir in light corn syrup and warm water until fully combined.

Pour warm glaze over the mousse carefully, tilting for even coverage. Let sit for about 1 hour at room temperature to set before slicing and serving.