Lightly grease an 8 x 8-inch baking pan and line with parchment paper. Leave a 2-inch overhang for easy unmoulding.
Add the water to the bottom of a large, high-sided, heavy-bottomed saucepan. Add the Redpath® Golden Yellow Sugar, corn syrup, and salt. Stir to thoroughly combine, being careful not to splash the sides of the pot with the sugar mixture as this may encourage the sugar to crystallize. Clip a candy thermometer to the side of the pan, ensuring the tip is in the mixture but not touching the bottom of the pan.
Place the saucepan over medium heat; do not stir when mixture is cooking. Bring the sugar mixture to 300°F (149°C) (hard crack stage). Use a clean pastry brush dipped in water to brush the sides of the pot if the mixture begins to crystallize.
Have the prepared pan and baking soda (see Chef’s Tips) nearby. Remove saucepan from heat and quickly whisk in the baking soda. Be careful, as the sugar mixture will begin to foam and rise in the pot.
Quickly pour the sponge mixture into the prepared pan and smooth the top with a heatproof spatula to create an even surface, being careful not to deflate the sponge mixture.
Let mixture cool completely at room temperature; 2 hours.
With the help of the parchment overhangs, lift and remove the completely cooled sponge toffee from the baking pan. Carefully peel off the parchment. Break sponge toffee into bite-size pieces using the handle of a wooden spoon or a rolling pin.