Preheat your oven to 350°F (175°C). In a mixing bowl, combine crushed chocolate graham crackers, melted butter, and granulated sugar. Press this mixture into the tart pan and bake for 10 minutes. Allow it to cool completely.

In a small bowl, sprinkle unflavored gelatin over cold water and let it sit for 5 minutes. Microwave for about 10 seconds until dissolved and set aside to cool slightly.

Melt white chocolate chips over a double boiler, then whisk in milk or cream, followed by dissolved gelatin and peppermint oil. Allow to cool below 85°F.

In a separate bowl, beat heavy whipping cream and powdered sugar until medium-stiff peaks form. Be careful not to overwhip.

Gently fold whipped cream into the cooled chocolate mixture and spoon into the tart shell. Cover and refrigerate for at least 4 hours or overnight.

Heat cream in a saucepan until simmering. Remove from heat and add chopped bittersweet chocolate, stirring until smooth. Mix in corn syrup and warm water to achieve a pourable consistency. Cool slightly.

Pour the chocolate glaze over the mousse tart and let it set at room temperature for about 1 hour. Serve chilled, garnished with fresh berries or crushed candy canes if desired.