Fill the water bath canner and heat it to medium-high. Place the jars in simmering water until ready to use. Cut and clean the strawberries, then place them in a large saucepan with lemon juice and butter. Gradually stir in the pectin. Bring the mixture to a rolling boil, mashing the strawberries to your desired consistency. Add sugar and stir until it dissolves completely. Boil hard for 2 minutes, stirring constantly. Remove from heat.

Ladle the hot jam into the hot jars, leaving 1/4 inch headspace. Wipe the rims clean, center the lids on the jars, and apply the bands until they are tight. Process the jars in the boiling water canner for 10 minutes. Remove the jars and cool them on the counter for 12-24 hours. The lids should "pop" as they seal and should not flex up and down when the center is pressed.

This recipe yields about 4 pint jars or 8 half-pint jars.