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.
Place about 2 inches of water into a medium heavy-bottomed saucepan. Bring water to a boil. Reduce heat to low. Place roughly chopped chocolate into a medium heatproof bowl. Fit the bowl over the pot of just simmering water, ensuring the bottom of the bowl does not touch the surface of the water. With a heatproof spatula, gently stir chocolate until almost completely melted. Carefully remove bowl from the pot, wiping the bottom of the bowl of any condensation with a kitchen towel. Stir chocolate until fully melted and smooth; the residual heat will further melt any chocolate that may not have melted.
Line a baking sheet with parchment paper. Set aside.
Dip each piece into the melted chocolate and place onto the parchment-lined baking sheet. Sprinkle with flaky sea salt, if desired. Allow chocolate to set; about 15 minutes.
Store in an airtight container layered between pieces of parchment paper at room temperature; do not place into the fridge (see Chef’s Tips).
*Have everything prepared in advance, as you will need to work quickly when the sugar mixture comes to temperature.
*Make sure your thermometer is calibrated. A few degrees off will lead to the sponge toffee either getting too hard or too soft or chewy. To check if your thermometer is calibrated, fill a glass at least halfway with ice and add cold water until the glass is almost full. Insert the probe into the ice water and stir slightly. The probe should not be touching the bottom or sides of the glass. After a minute, check the temperature on the thermometer. If the temperature reads 32°F (0°C), your thermometer is calibrated.
*To ensure there are no lumps of baking soda going into the candy, sift or whisk the baking soda in a small bowl. When adding to the mixture, sprinkle the baking soda over the surface of the mixture versus adding it in one spot as you will be whisking the mixture very briefly.
*As a precaution, wear oven mitts when whisking the baking soda into the sugar mixture as it creates a lot of heat and steam.
*Sugar is hygroscopic; therefore, storing sponge toffee (and all candies) in the fridge is not recommended.
*To easily clean your saucepan, candy thermometer, and spatula of hardened sugar, add water to the dirty pot and bring to a boil. Clip your thermometer and place spatula into the boiling water. Turn off heat and allow the hot water to melt the sugar.