Grease a 20cm/8in square baking tin with vegetable oil.
Gently heat the butter, sugar and golden syrup in a large heavy-based pan until the sugar has dissolved. Turn up the heat and boil rapidly, without stirring. If the mixture goes darker at one side of the mix, then gently swirl the pan to mix it all together. Keep boiling until the mixture goes a golden-brown colour – this will take about 5 minutes.
Add the bicarbonate of soda and stir it for a few seconds, which will make the mixture expand and fizz (take care, the mixture will expand a lot). Tip the honeycomb into the oiled baking tin and leave until cold and set.
To serve, cut the honeycomb into pieces and stir into ice creams or decorate puddings. You can put the honeycomb in a food processor and sprinkle it over desserts.
To stop the honeycomb mixture from burning, dip a pastry brush in some water and brush the sides of the pan to remove any sugar.