Place a couple of plates/saucers into a freezer.We'll be using these to test if the jam is cooked enough, Peel & core the apples then cut into 2 cm pieces.Add into a large saucepan with the 150ml of water. Place the saucepan over a low heat & cook until the apples are starting to soften. This will take around 10 minutes.
Next, stir in the blackberries & continue cooking over a low heat until all the fruit has softened. Another 10 minutes, roughly.
Add in the lemon juice & sugar then keep cooking over a low heat until the sugar has fully dissolved. Make sure to stir regularly.
Once the sugar has completely dissolved, turn the heat up to medium-high & bring to a rapid boil.
Boil for 5 minutes, stirring as little as possible then remove from the heat.Spoon a small amount of jam onto one of the cold plates & leave for 1 minutes. Drag a finger or spoon through the jam. If it wrinkles, it's ready. If it doesn't, return the jam to the heat & cook for another couple of minutes then test again. Once the jam is cooked, remove from the heat & stir in the butter, vanilla & salt.
Leave to cool slightly then transfer into sterilised jars/containers.