Preheat oven to 175°C (350°F) and bake a prepared tart shell until lightly golden. Allow it to cool completely.
Heat heavy cream in a saucepan until just simmering, then pour over chopped dark chocolate.
Stir until smooth to create a glossy chocolate ganache.
Pour the ganache into the cooled tart shell and smooth the surface evenly.
Melt white chocolate and transfer it to a piping bag or small zip bag.
Pipe concentric circles over the ganache, then drag a toothpick from the center outward to form a spider web pattern.
Chill the tart for 2–3 hours until set, then decorate with edible spiders or Halloween sprinkles before serving.