Grease a 20cm square tin with butter. Melt the sugar and golden syrup together in a heavy based pan over a low heat. Increase the heat to medium and simmer for 3-4 minutes, until the mixture is thick and a dark caramel colour. Remove from the heat and immediately whisk in the bicarbonate of soda so that the mixture froths up. Pour into the tin and leave to set at room temperature for about 2-3 hours.
Grease a large sheet of foil with the vegetable oil. Once set, bash the toffee mixture into large shards/splinters. Melt the chocolate in a microwave oven or in a bowl over a pan of simmering (not boiling) water. Dip the shards of toffee into the melted chocolate and place them on a lightly oiled foil sheet. Leave to set in the fridge for about 20 minutes.