Popularly called as molasses/dark caramel/burnt sugar, treacle is a byproduct of the process of making sugar. You can make a cheat's version by caramelising sugar to produce a deep black, bitter-sweet thick syrup that is widely used to flavour and colour cakes, fruitcakes, puddings, tarts, sauces and marinades
Once the boiling water has been added, stir the mixture and then add the 2 cups of sugar and mix it with a ladle till it is dissolved and turns into liquid. Cook on a low heat till the syrup thickens a little.
Add the lime/lemon juice to prevent crystallization. Reduce the heat and cook for another 8-10 minutes or slightly more.
Do not be tempted to overcook the syrup as it can quickly turn into a hardball consistency (required to make candy/hard boiled sweets) and if you pour it into a jar, once it cools you won't be able to get it out. You will then need to add water to the jar to even soften it.
If the syrup has thickened a bit too much it will start turning into a string like consistency and when cooled, can be modelled. This is a soft ball stage. In such a case, just add a little water and bring it back to a simmer until it passes the consistency test as mentioned in the points above.