Put a large saucepan over medium heat and add in the rhubarb, sugar, lemon zest (but not the lemon juice) and salt.
Cook, stirring occasionally, for 8-10 minutes, or until the mixture has started to boil and thickens a bit.
Cover the saucepan with a lit and let it to simmer for 10 minutes, stirring occasionally, to thicken even more.
If the jam hasn’t thickened enough to coat the back of a spoon after 20 minutes, then stir in 2 teaspoons cornstarch and simmer for another 5 minutes (but not longer, as it might break down the whole mixture into a runny mess).
Once the jam has thickened, remove the pan from the heat and stir in the lemon juice.
Transfer the jam to a container (I prefer a wide one, so that jam cools quickly) and allow it to cool completely in the fridge (about 2-3 hours, depending on the size of your container).
While the jam is cooling, make the cupcakes.