Preheat your oven to 350°F (175°C). In a mixing bowl, combine chocolate graham cracker crumbs, melted butter, and granulated sugar until well mixed. Press this mixture firmly into the bottom and up the sides of a 9.5" tart pan. Bake for 10 minutes until slightly firm, then remove from the oven and cool completely before filling.

In a small bowl, sprinkle unflavored gelatin over 2 tablespoons of cold water. Allow it to sit for 5 minutes, letting the gelatin bloom. After blooming, microwave the mixture for about 10-15 seconds until just dissolved—ensure it doesn't boil. Set aside to cool slightly while you prepare the mousse.

Melt white chocolate chips in a heatproof bowl over simmering water, stirring until smooth. Remove from heat and mix in your choice of milk (or half and half). Slowly incorporate the dissolved gelatin and peppermint oil until blended. Let this mixture cool to below 85°F, stirring occasionally to maintain a smooth texture.

In a separate bowl, use an electric mixer to whip heavy whipping cream with powdered sugar until medium-stiff peaks form, which should take about 3-4 minutes. Gently fold the whipped cream into the cooled chocolate-peppermint mixture in two additions, being careful not to deflate the mixture to maintain its lightness and airiness.

Spoon the fluffy mousse filling into the cooled tart shell, using a spatula to smooth the top evenly. Cover the tart lightly with plastic wrap and refrigerate for a few hours or ideally overnight to let the mousse set fully and develop its flavors.

In a small saucepan, heat cream over medium heat until it comes to a gentle boil. Remove it from heat and stir in bittersweet or dark chocolate until melted and smooth. Mix in light corn syrup and very warm water, adjusting the consistency until smooth and glossy. Let cool slightly before pouring over the chilled mousse tart.

Carefully pour the chocolate glaze over the chilled mousse filling in the tart shell, tilting the pan gently to ensure even coverage. Allow the glaze to set at room temperature for about 1 hour. Once set, your Dark Chocolate Peppermint Mousse Tart will be ready to slice and serve, showcasing its stunning layers and delightful flavors.