Prepare a baking sheet by lining it with aluminum foil and spraying it with nonstick cooking spray.
Combine the butter, sugar, water and corn syrup in a large heavy saucepan over medium heat. Stir constantly to dissolve the sugar and melt the butter. Insert a candy thermometer and bring the candy to a boil, periodically brushing down the sides with a wet pastry brush to prevent crystallization.
Continue to cook the candy, stirring frequently, until it reaches 300 degrees. If the candy appears to separate (with a layer of melted butter on top) stir vigorously to make it come back together again. Watch the candy as it approaches 300, since it cooks quickly and can scorch at high temperatures.
Once the candy reaches 300 degrees, remove from heat and pour it onto the prepared baking sheet. Use a spatula or spoon to spread the candy to an even thickness. allow the toffee to begin to set, then spray a knife with nonstick cooking spray and score the toffee into small squares or rectangles. allow the toffee to cool completely.
Once the toffee is cool, break it apart on the scored lines into small squares. The toffee can be left plain, or dipped in chocolate.
To make toffee baking pieces, place the toffee in a ziploc bag and crush using a wooden mallet or rolling pin until you have roughly uniform pieces.
