1. Preheat your oven to 350°F (175°C). Prepare a 9-inch springform pan by lightly greasing it or lining it with parchment paper.
2. Make the crust: In a medium bowl, mix together the graham cracker crumbs, 1/4 cup sugar, and melted butter until well combined. Press the mixture firmly into the bottom of the prepared pan. Bake for 10 minutes, then remove from the oven and let cool.
3. Prepare the filling: In a large bowl, beat the cream cheese and 1 cup sugar until smooth. Add eggs, one at a time, fully incorporating each before adding the next. Gently fold in the melted dark chocolate, orange zest, and orange liqueur (if using) until well combined.
4. Bake the cheesecake: Pour the filling over the prepared crust and smooth the top with a spatula. Place the springform pan in a larger baking pan. Fill the larger pan with hot water halfway up the sides of the springform pan to create a water bath. Bake for 55-60 minutes, or until the center is just set and the top appears dull.
5. Cool the cheesecake: Remove the cheesecake from the water bath and run a knife around the edges to loosen it from the pan. Allow it to cool to room temperature, then refrigerate for at least 4 hours, preferably overnight.
6. Prepare the topping: For the ganache, heat the heavy cream in a small saucepan over medium heat until it begins to simmer. Remove from heat and pour over the chopped dark chocolate. Let it sit for a minute, then stir until smooth and glossy. Pour the ganache over the chilled cheesecake, spreading it evenly.
7. Garnish and serve: Sprinkle additional orange zest and arrange orange slices on top for garnish. Let the cheesecake set in the fridge for an hour before serving.

Notes

• Orange Liqueur Alternative: If you prefer not to use alcohol, replace the orange liqueur with orange juice or an additional tablespoon of orange zest for a stronger orange flavor.
• Serving Suggestion: For an extra touch of elegance, serve each slice with a dollop of whipped cream and a drizzle of chocolate sauce.