In a food processor, combine plain flour and icing sugar, pulsing until well-mixed. Add cold, cubed unsalted butter and pulse until the mixture resembles coarse crumbs. Crack in an egg, mixing until a dough forms. Wrap the dough in plastic wrap and chill in the refrigerator for about 30 minutes to firm up, ensuring a flaky texture.

Preheat your oven to 180°C (350°F). On a lightly floured surface, roll out the chilled dough to about 3mm thick. Carefully press the dough into your tartlet molds, trimming the excess. Bake for 12-15 minutes, or until golden brown and fragrant, slightly puffed up. Once baked, let the tart shells cool completely on a wire rack.

In a large mixing bowl, beat the softened full-fat cream cheese and caster sugar together until smooth and creamy. Gently fold in the lemon zest for brightness, then whisk in the thickened cream until fully combined. Gradually add ¾ cup of pistachio paste, stirring until the filling is a rich, vibrant green, and all ingredients are well incorporated.

Once the tart shells are cool, carefully spoon or pipe the pistachio cheesecake filling into each shell, ensuring they're filled generously. For an exciting surprise, place a small ball of reserved pistachio paste at the bottom of the filling in each shell.

Melt dark chocolate gently in a microwave or double boiler, stirring until smooth and glossy. Drizzle the melted chocolate artistically over the filled tartlets for a beautiful finish. Optionally, add decorative touches like rose petals or fresh raspberries atop the tartlets. Chill in the refrigerator for at least 6 hours, allowing the filling to set well and the flavors to meld.