Step 1: Prepare your canning equipment by washing jars, lids, and rings in hot, soapy water. Keep the jars warm in a large pot of simmering water (the canner) until ready to use.
Step 2: Wash, hull, and crush the strawberries one layer at a time using a potato masher until you have exactly 5 cups of crushed fruit.
Step 3: In a large, heavy-bottomed pot, combine the crushed strawberries, lemon juice, and the powdered pectin. Stir well until the pectin is fully dissolved.
Step 4: Bring the mixture to a full rolling boil over high heat, stirring constantly to prevent burning.
Step 5: Stir in the sugar all at once. Add the butter if using to minimize foam. Return the mixture to a full rolling boil and boil hard for exactly 1 minute, stirring constantly.
Step 6: Remove the pot from the heat and skim off any remaining foam with a metal spoon. Quickly ladle the hot jam into the warm jars, leaving 1/4 inch of headspace at the top.
Step 7: Wipe the rims of the jars with a clean, damp cloth. Center the lids on the jars and screw the bands on until fingertip tight. Process in a boiling water canner for 10 minutes (adjust for altitude).
Step 8: Remove jars from the canner and place them on a towel-lined counter to cool for 24 hours without moving. Check the seals by pressing the center of the lid; it should not pop back.