Prepare a square baking dish by greasing it lightly and dusting with cornstarch.
In a saucepan, combine sugar and water, bringing it to a boil over medium heat.
Add lemon juice and continue boiling until it reaches a soft-ball stage.
In another bowl, mix cornstarch with 1 cup of water to create a slurry.
Gradually add the cornstarch mixture to the boiling sugar mixture while stirring constantly.
Reduce heat and cook for about 30 minutes, stirring frequently until thickened.
Stir in rosewater and food coloring if desired.
Pour the mixture into the prepared baking dish and smooth the top.
Let it cool at room temperature for several hours or overnight.
Once set, cut into squares and dust with powdered sugar before serving.