Prepare a 9″ x 13″ cake pan with nonstick spray, or a light coating of vegetable oil or shortening. Set aside.
In a LARGE pot (it will bubble up like mad as it cooks!), 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. Continue beating the mixture until the foaming starts to slow down.
Dump foaming mixture into greased cake pan, spreading it out as evenly as possible. Allow it to cool for 15-20 minutes.
When the sponge toffee is starting to harden – but is still quite warm – use a serrated knife to score lines, about 1/4″- 1/2″ deep in the warm candy. These will be the shapes of your candy bars – I like to make them about 1.25″ x 3″, or so. Keep in mind that this is not an exact science, and you WILL have breakage in there.
20 minutes later, go back and re-score the lines you already made, gently cutting a little deeper than last time. Allow to cool completely.
Once toffee is cooled all the way through, remove from pan and gently snap along your score lines. If you don’t plan to dip them right away, be sure to store toffee in an airtight container – the sugar will attract water from the air, and the toffee can go soggy.
In a large bowl, carefully melt your chocolate using whatever method you prefer. I like to use a glass bowl, nuking for 30 seconds at a time in the microwave, many swear by a double boiler. So long as you don’t scorch it, it’s all good.
Using a fondue fork, candy dipping utensil, or (Clean!) fingers, gently dip each piece of sponge toffee, swirling around to coat fully. Allow excess chocolate to drip off before placing each piece on wax paper, parchment, or foil to harden.