In a heavy-bottomed, medium saucepan, combine the pitted prunes, water, orange juice, lemon juice, sugar, and salt.
Bring the mixture to a rolling boil over medium-high heat.
Reduce the heat to low, cover, and let it gently simmer for 30 minutes, allowing the prunes to soften completely.
Carefully transfer the hot mixture in batches to a blender or food processor.
Blend until the mixture is completely smooth and puréed.
Return the smooth purée to the saucepan and continue to simmer on low heat, uncovered, for another 20-30 minutes.
Stir frequently to prevent it from sticking and scorching as it thickens.
To check if it's ready, perform a plate test.
Place a small plate in the freezer for a few minutes.
Drop a spoonful of the hot butter onto the cold plate and return it to the freezer for 5 minutes. If a watery rim does not separate from the butter, it is perfectly thickened. If it does, continue cooking for another 10 minutes and test again.
Once done, remove the pan from the heat and let the prune butter cool to lukewarm. Spoon it into clean, airtight glass jars or containers. It will keep in the refrigerator for several months.