Peel and pit the mangoes. Measure exactly 2 cups of the prepared fruit into a 2- or 4-quart pot.
Grind or crush the fruit with a potato masher. Stir in the lemon juice.
Place the mixture on the stove and cook over medium heat, stirring regularly.
After about 4-5 minutes,* add the sugar and butter to the pot.
Bring the mixture to full rolling boil on high heat, stirring constantly.
Stir in the pectin. Return the mixture to full rolling boil and boil 1 minute, stirring constantly. Remove from heat. Skim off any foam with a spoon.
Ladle the mixture immediately into storage container.
After it cools, you are ready to eat your mango jam! Try it on toast or with crackers and cream cheese. Store in a covered container and refrigerate. It will last 1-2 weeks.
Servings 15