Sterilize your canning jars and lids according to manufacturer's instructions or by boiling them in water for at least 10 minutes. Keep them hot until ready to fill.

Gently wash the mulberries under cool running water and meticulously remove any remaining stems or debris. Place the cleaned mulberries in a large, heavy-bottomed non-reactive pot or Dutch oven.

Using a potato masher or a sturdy fork, gently mash the mulberries to release their juices. You can leave some fruit whole for a chunkier jam or mash more thoroughly for a smoother texture.

Add the powdered fruit pectin and freshly squeezed lemon juice to the mashed mulberries in the pot. Stir well to combine these ingredients thoroughly.

Place the pot over high heat and bring the mixture to a full rolling boil (a boil that cannot be stirred down). Stir constantly to prevent sticking and ensure even heating.

Once at a full rolling boil, add the granulated sugar all at once. Stir continuously until the sugar is completely dissolved. If using, add the optional unsalted butter at this stage to help reduce foam formation.

Return the mixture to a full rolling boil and boil hard for exactly 1 minute, stirring constantly to avoid scorching the bottom of the pot.

After 1 minute of hard boiling, remove the pot from the heat. To test for set, place a small spoonful of jam onto a chilled plate. Let it sit for 30 seconds, then push the jam with your finger. If it wrinkles, it's ready. If not, return to heat and boil for another minute, then retest.

If desired, skim off any foam that has accumulated on the surface of the jam using a large spoon. This improves clarity.

Carefully ladle the hot jam into the hot, sterilized jars, leaving 1/4 inch headspace from the rim. Use a canning funnel for easier and cleaner filling. Wipe jar rims clean with a damp cloth to ensure a proper seal.

Center new lids on the filled jars and apply bands, tightening just until finger-tight. Do not overtighten.

For shelf stability, process the filled jars in a boiling water canner. Place jars on a canning rack in a large canner or stockpot. Ensure jars are covered by at least 1 inch of water. Bring to a full rolling boil and process for 10 minutes (adjust for altitude if necessary: add 1 minute for every 1,000 feet above sea level).

Carefully remove jars from the canner using a jar lifter and place them upright on a clean kitchen towel or wire rack to cool completely, undisturbed, for 12-24 hours. You should hear a 'pop' as the jars seal.

After cooling, check the seals by pressing the center of the lid; it should not flex or pop up. If a jar doesn't seal, refrigerate it and use within 3 weeks. Store properly sealed jars in a cool, dark place for up to 1 year.