Once the chocolate-coated balls have set, spoon the leftover melted dark chocolate into a piping bag fitted with a fine nozzle.
On a parchment-lined tray, pipe 24 small circles, 1 per spider, each with 8 legs (4 on each side).
Place a chocolate-coated ball gently onto each circle of melted chocolate while the chocolate is still wet, so they fuse as they set. Chill these while you melt the white chocolate for the eyes.
Melt the white chocolate in a bowl set over the saucepan of simmering water.
Let the melted white chocolate cool for about 10 minutes, until it’s still melted but cool to the touch, then spoon it into a piping bag fitted with a fine nozzle.
Pipe two white chocolate eyes onto each spider.
To add the pupils, use a toothpick or the piping bag to dot a little dark chocolate onto the centre of the white chocolate eyes.Note: If the chocolate is too runny to pipe, give it another 10 minutes or so to cool at room temperature - it thickens as it cools, which makes piping a little easier. Chill the spiders until fully set, then carefully peel the spiders off the parchment. Store them carefully in the fridge in a single layer.