Preheat your oven to 140°C (275°F)/fan 120°C (250°F)/gas mark 1 and line two large baking trays with baking parchment.
For the biscuits, combine the flour, bicarbonate of soda, sugar, finely chopped chocolate, cocoa powder, and salt in a large mixing bowl or a food mixer with the beater attachment.
Add the softened butter and golden syrup, beating until the mixture is thoroughly combined into a dough.
Divide the dough into 16 equal balls (about 1 heaped tbsp each).
Place them evenly spaced over the two prepared baking trays.
Bake for 25–30 minutes, turning the trays halfway through to ensure they cook evenly.
Once baked, place the trays on a cooling rack and allow the biscuits to cool completely.
While the biscuits cool, make the filling. Melt the 75g (2.65 oz) of chopped chocolate either in a microwave on a low setting in short bursts, or in a heatproof bowl set over a pan of simmering water (don't let the bowl touch the water).
Stir the popped rice cereal into the melted chocolate until every piece is completely coated.
To assemble, take two cooled biscuits.
Turn them upside-down.
Spread 1 tbsp (15 ml) of the chocolate cereal filling on one biscuit and 1 tbsp (15 ml) of salted butterscotch on the other, then gently sandwich them together.
Repeat with the remaining biscuits.
Leave the assembled biscuits to rest for at least 30 minutes to allow the filling to set before serving.
Don't overbake the biscuits; they should still be slightly soft in the centre when you remove them from the oven as they will firm up upon cooling.