If water-bath canning, prepare mason jars before cooking by submerging them in hot water to sterilize while you make the jam.
Mash the strawberries and mangos with a food processor or potato masher until the fruit is in small chunks. Measure 3 cups mashed mango and 3 cups mashed strawberries.
Add the mashed mango, mashed strawberries, powdered pectin, and lemon juice to a large pot. Place over high heat and bring the fruit mixture to a rolling boil.
Add the optional pat of butter and stir until melted and fully incorporated into the fruit mixture.
Add the sugar and continue cooking over high heat. Stir often to prevent scorching until the jam reaches a hard boil.
Boil the jam hard for 1 to 2 minutes, then turn off the heat.
Skim off excess foam from the top of the jam with a spoon.
Let the jam cool slightly, then ladle it into prepared jars.
For canning, process the filled jars in a water bath for 15 minutes, adjusting for altitude as needed.
For refrigerator jam, let the jars cool to room temperature, then refrigerate and use within 2 weeks.
