Place the graham crackers in a zip-top bag and crush them into fine crumbs using a rolling pin or your hands. Transfer to a bowl, then pour in the melted butter and mix until the crumbs are evenly coated and resemble wet sand.
Press the crumb mixture firmly into the bottom of a springform pan, spreading it out evenly with the back of a spoon or your fingers. Chill the crust in the fridge for 15 minutes to set.
In a heatproof bowl, melt the chopped dark chocolate over a saucepan of simmering water or in short bursts in the microwave, stirring until smooth and glossy. Let it cool slightly so it doesn’t seize the cream cheese.
In a mixing bowl, beat the softened cream cheese until smooth and creamy, about 2 minutes. Gently fold in the cooled melted chocolate until fully combined.
In a chilled bowl, whip the heavy cream, sugar, vanilla extract, and lemon zest (if using) until soft peaks form, about 3-4 minutes. Be careful not to over-whip to prevent graininess.
Gently fold the whipped cream into the cream cheese and chocolate mixture in three additions, combining carefully to keep the mixture light and airy.
Pour the smooth filling over the chilled crust, spreading it evenly with a spatula. Tap the pan lightly on the counter to release any air bubbles and create a flat surface.
Cover the pan loosely with plastic wrap and refrigerate for at least 4 hours, preferably overnight, until the filling is firm and set with a glossy finish.
Once set, carefully remove the sides of the springform pan. Use a sharp, warm knife to slice cleanly through the cheesecake, wiping the blade between cuts for the neatest slices.
Garnish as desired with whipped cream, chocolate shavings, or fresh berries. Serve and enjoy this rich, silky, no-bake chocolate cheesecake!
