Preheat oven to 300F (140C). Grease an 9" square baking pan generously with butter.
In a saucepan, melt together butter, brown sugar and golden syrup. Stir in salt and cocoa powder until smooth. Add oats, raisins and nuts. Stir until oats are well-coated with chocolate sauce.
Pour into pan and press smooth with a fork or spoon. Bake 15-20 minutes until just cooked through.
Place pan on a cooling rack for 20 minutes. Cut into 3" wide strips, then turn pan 90° and cut 1" wide strips. (You should have 27 pieces.) Allow to cool a further 20 minutes, then turn out bars.
Place chocolate bits into a microwave-safe coffee mug; heat on 70% power for 60 seconds. Stir. Repeat using 15 second pulses then stirring, until fully melted. Dip half of each bar in chocolate, then place onto Silpat, aluminum foil sprayed with non-stick spray, or baker's parchment. When completely cool and set (about 1 hour), store in airtight container at room temperature up to a week.