Fill a large pot or water bath canner with enough water to fully cover the mason jars. Place a lid on top of the large pot and bring the water to a boil and carefully place the jars into the pot. Boil for 10 minutes to sterilize the jars. Place the lids in hot water until ready to use.
While the mason jars are coming up to boil, wash your strawberries thoroughly, remove the tops, and chop into small pieces. Add the strawberries to a large bowl and mash using a potato masher until the berries reach your desired consistency.
In a separate large pot, combine the mashed strawberries, pectin, butter, and lemon juice. Do not turn on the heat yet.
Once the jars begin boiling, turn the strawberry mixture to high heat. Stir occasionally as the strawberries begin to simmer and bubble around the edges. After several minutes, add the sugar and stir until fully dissolved. Continue cooking over high heat, stirring periodically, until the jam reaches a full rolling boil that cannot be stirred down. Once the jam reaches a rolling boil, continue boiling for 1 minute while stirring continuously.
Turn off the heat and skim off any foam from the top of the jam using a spoon. Using a canning lifter, carefully remove the hot sterilized jars from their water bath and place on a heat safe surface like a wooden cutting board. Next, using a canning funnel, carefully ladle the hot jam into sterilized jars. Take care to leave about ½ inch of space at the top of the jar. Wipe the rim clean with a damp paper towel before sealing. Place the lids on the jars and screw the bands on tightly. Then carefully return the filled jars to the hot water bath. Ensure the jars are fully submerged and process for 10 minutes. Place a lid on top of your large pot.
Turn off the heat, remove the lid from the pot, and allow the jars to rest in the hot water for 5 minutes. Remove the jars carefully using a canning lifter and place them on a wooden cutting board or towel at room temperature. Allow the jars to sit undisturbed for at least 12 hours at room temperature.
Once sealed, store in a cool dry place. Refrigerate after opening and enjoy over biscuits, toast, peanut butter sandwiches, or ice cream!