Line a large baking tray with baking paper. Cook the popcorn according to pack instructions.
Set a large heavy-based saucepan with the butter, sugar and syrup over a medium-low heat. Leave to melt, then increase the heat to medium. Simmer for 3 minutes, without stirring, until it is a dark caramel colour.
Stir in a pinch of salt, the bicarbonate of soda, ginger and ginger biscuits – the mixture will bubble.
Quickly add the popcorn, stir well to coat, then tip onto the tray, separating with a fork. Once the popcorn is cool enough to handle, break apart any that are stuck together. Leave to cool and set completely before serving.