Makes 1 tray, 24-36 portions.
Grease and line a 9-x-13-inch (23-x-33 cm) pan with parchment paper so that the paper comes up higher than the sides.
Have the honey measured in one dish and stir the baking soda and cream of tartar together in another dish. Have these on hand, along with a sieve, by the stove before you start cooking the sugar.
In a large* saucepan, place the water (water first allows the sugar to dissolve evenly), sugar and corn syrup and bring the mixture to a boil over high heat, stirring just once or twice to ensure everything is dissolving evenly. Continue boiling, without stirring, until the mixture reaches 284ºF (140ºC) on a candy thermometer. At this point, stir in the honey, and continue boiling until 300ºF (150ºC) is reached (the liquid will only be a pale amber colour, not caramelized fully). Remove the pot from the heat and using a wooden spoon quickly sift in the baking soda mixture, stirring quickly and vigorously to make sure it has been incorporated, but once the baking soda is no longer visible, stop stirring so as not to deflate the toffee. Quickly scrape the frothing mass (it will keep growing and growing!) into the prepared pan and let it set (do not spread it) until cool – this can take up to 2 hours, and it will continue to grow a little more before it sets.
Crack the toffee with a rolling pin to break into bite-sized pieces.
The toffee will keep up to 4 weeks in an airtight container.
*You need a large saucepan to factor in room for the sponge toffee to expand.