Make the crust
Preheat oven to 350°F (175°C). Mix the chocolate graham crumbs with melted butter and sugar. Press the mixture firmly into the base and up the sides of a 9.5″ × 1″ tart pan with a removable bottom. Bake about 10 minutes. Let cool on a rack for at least 15 minutes.

Prepare the gelatin
Sprinkle the gelatin over the cold water and let it sit for 5 minutes to soften. Heat the bloomed gelatin in the microwave for 15–30 seconds just until it dissolves. Do not boil.

Build the white-chocolate base
Melt the white chocolate in short microwave intervals (30 seconds), stirring between bursts until smooth. Warm the milk until it’s steaming (about 30 seconds), then whisk it into the melted white chocolate until fully combined. Stir the dissolved gelatin and the peppermint oil into the mixture and whisk until uniform. Allow the chocolate blend to cool to 85°F (about) or lower, but don’t let it harden.

Whip the cream
Beat the heavy cream and powdered sugar until medium-stiff peaks form. Fold one-third of the whipped cream into the cooled white-chocolate mixture to loosen it, then gently fold in the remaining cream until the mousse is uniform and airy. Fold carefully — overmixing will deflate the mousse.

Assemble and chill
Spoon the mousse into the cooled crust and smooth the surface with an offset spatula. Cover and chill in the fridge for several hours or overnight so the mousse sets firmly.

Make the glaze
When the tart is completely chilled, prepare the glaze. Heat the cream until it just comes to a boil, then remove from heat and stir in the chopped dark chocolate until smooth. Add the corn syrup and stir, then incorporate the very warm water to reach a pourable consistency. Let the glaze cool slightly so it’s pourable but not scalding.

Glaze the tart
Pour the glaze onto the center of the mousse-filled tart and tilt the pan so the chocolate spreads evenly across the top. The surface should be fully covered with no white showing. Allow the glaze to set at room temperature for about 1 hour.