Line a 9"x13" pan with foil.
Combine the sugar and syrup in a heavy medium saucepan set over medium-low heat. (Make sure there is lots of room in the pan for the mixture to foam up when you stir in the baking soda.) Stir until the sugar begins to melt. Continue to cook, swirling the pan occasionally but not stirring, until the mixture turns a deep amber. Watch it carefully – sugar burns fast!
Remove from heat and quickly stir in the baking soda. It will foam up like a science experiment! Quickly pour it into the pan and set aside for an hour, or until set. Peel off the foil and break into chunks to serve.