Preheat the oven to 170°C, fan 150°C, gas 3. Grease and line a 20cm square fixed-base cake tin. Put the butter, caster sugar and syrup in a heavy-based saucepan and place over a medium low heat. Stir the mixture until it’s smooth and the sugar has almost melted. Remove from the heat and stir in the chocolate until smooth. Pour into a bowl and cool for 20 minutes until lukewarm, then mix in the eggs and vanilla. Sift in the flour and a pinch of salt, and fold through until just combined.

Pour half of the mixture into the tin and use a tablespoon to dollop over 16 equally sized spoonfuls of the salted caramel. Sprinkle on the shards of 2 Crunchie bars, then pour the remaining brownie mixture over the top to cover. Bake for 40-45 minutes. The brownie will still have a wobble to it, but will set during cooling. Sprinkle over the remaining Crunchie shards if using and press in very lightly.

Cool in the tin for 20 minutes, then transfer to the fridge for 2 hours to set before slicing.