Preheat oven to 350°F (175°C). Mix crushed chocolate graham crackers, melted butter, and sugar until resembles wet sand. Press into tart pan. Bake for 10 minutes, let cool.

Bloom gelatin by sprinkling it over cold water and letting sit for 5 minutes. Melt white chocolate over simmering water, stir in warmed milk. Combine with gelatin and peppermint oil, let cool slightly.

Whip heavy cream and powdered sugar until medium-stiff peaks. Fold whipped cream into the cooled white chocolate mixture. Spoon into tart shell and refrigerate until set.

Heat heavy cream until boiling, stir in chopped chocolate until melted and smooth. Add corn syrup and warm water, stirring until glossy. Let cool slightly.

Pour warm glaze over set mousse, tilting to spread evenly. Let set at room temperature for about an hour before slicing.