Wash and prepare the cherries. If you're using frozen, there is no need to thaw them first.
In a medium saucepan, add the cherries, granulated sugar, fresh lemon juice & zest.
Mix over medium-low heat until sugar dissolves (about 5 min).
Increase the heat and bring mixture to a boil and allow to boil for 3 minutes, then remove from the heat and stir in the butter.
Return to the heat, and bring to a boil. Then lower the heat to medium. Stir and mash the cherries frequently and continue to boil until the jam has thickened (or until it reaches a temperature of 220°F/104°C) This will take approximately 10 to 15 minutes.
Leave to cool slightly and carfully transfer the jam into a clean, tempered jar.