Place the sugar, lemon juice, and 185ml (¾ cup) of the water into a heavy-based pan over a medium heat.
Stir constantly, until the sugar has dissolved, bring to the boil, and brush down the inside of the pan with cold water (this will stop crystals forming). Insert the thermometer - don’t let it rest on the bottom of the pan though, or you’ll get a false reading.
Boil the syrup (keep stirring), until it reaches 115°C (240°F). (This is the soft ball stage. If you’re not using a thermometer, you will need to test the syrup by dropping a teaspoonful into a glass of cold water. If it forms a soft ball, the syrup is ready. It should take anywhere between 10-15 minutes to reach this stage.)
Remove the pan from the heat. The syrup will be a light gold colour.
Make the Turkish Delight:
While the syrup is cooling slightly, place the rest of the water (250ml / 1 cup + 2 tsp) into another heavy-based pan over a medium heat, along with the cream of tartar (to stop the sugar crystallising), and the cornflour. Whisk to make sure there are no lumps.
Whisk the mixture constantly, until it boils, and you have a very thick paste. Turn off the heat.
Carefully, and slowly, pour the syrup into the cornflour paste, a little at a time, making sure to keep whisking. Once you’ve added in all the syrup, whisk a little more to ensure that there are no lumps, and you have a smooth, golden, semi-translucent paste.
Scrape down the sides of the pan, or else you’ll get crusty stuff falling into the paste.
Over a medium heat, and stirring all the time, slowly bring the paste to a boil.
Turn the heat down to its lowest setting, and gently simmer for an hour or so, stirring every few minutes to prevent sticking. The paste will now be a deep golden colour. Turn off the heat, mix in the rosewater and food colouring (if using).
Oil the dish or baking tin, and line with baking parchment. Oil the baking parchment.
Pour the Turkish Delight into the prepared dish, and give it a bit of a shake to ensure the mixture gets right into the corners.
Cover with a piece of oiled baking parchment, and leave for around five to six hours to cool and set.
Finish the Turkish Delight:
When it’s ready, it will feel firm, and slightly tacky on top.
Sift together the icing sugar and the rest of the cornflour into a large bowl, then sieve some of this over the surface of the Turkish Delight, until it’s all covered.
Sieve more icing sugar/cornflour over a large chopping board or work surface.
Turn out the Turkish Delight onto the powder, and gently peel away the baking parchment. Sieve more of the icing sugar mix over the surface.
Using a large oiled knife, cut the Turkish Delight into 36 cubes. I’ve been told that a pizza cutter works really well for this.
Place each cube of Turkish Delight into the bowl with the rest of the icing sugar mix, and toss them around to ensure they are all well-coated.
Store at room temperature in between sheets of baking parchment, and covered with the rest of the icing sugar/cornflour mixture in a container with a loosely-fitting lid.
Don’t refrigerate, or else the Turkish Delight will sweat, and become a nasty, gooey mess!
This will keep for a good couple of months; however, the longer it's kept, the more chewy it will become. If you want it to be soft, eat within a week or two.