Prepare a 9″ x 13″ cake pan with nonstick spray, or a light coating of vegetable oil or shortening. Set aside.
In a large saucepan, stir together sugar, corn syrup, and water. Attach a candy thermometer to the pan, making sure that it does not touch the bottom of the pan.
Bring mixture to a boil, and allow to cook until temperature reaches 300 degrees F (hard crack stage). From the time mixture starts boiling to the time it reaches 300F, do not stir.
Once mixture reaches 300F, remove from heat.
Add vanilla and baking soda, beating to incorporate. The mixture will start foaming quite a bit when you add the baking soda (chemical reaction!), so using a LONG wooden spoon is a good idea. The sugar will be very hot, and will burn if you get any on your hand as you stir. Be very careful and work FAST.
Dump foaming mixture into greased cake pan, spreading it out as evenly as possible. Allow it to cool completely.
Once toffee is cooled all the way through, remove from pan and snap into chunks / nuggets and serve.