Wash your pears under cool water and pat them dry. Peel each pear with a vegetable peeler or knife, then cut them in half lengthwise and scoop out the core and seeds with a small spoon or melon baller. Chop the pears into rough 1-inch chunks. You should have about 10 to 12 cups of chopped pears.

Pour the chopped pears into your large pot and add the sugar, lemon juice, cinnamon, ginger, cloves, nutmeg, and salt. Stir everything together until the sugar begins to dissolve and coat the pears.

Set the pot over medium heat and bring the mixture to a gentle simmer, stirring occasionally. This usually takes about 15 to 20 minutes, and you'll see liquid accumulating at the bottom of the pot.

Once simmering, reduce the heat to low and add your split vanilla bean (if using the bean rather than extract). Stir the mixture every 10 to 15 minutes to prevent sticking and ensure even cooking. Continue cooking for about 1.5 to 2 hours. The mixture should bubble very gently, and the pears will gradually break down and become very soft.

After about 2 hours, use a wooden spoon to mash the pears against the side of the pot. They should break down easily into a rough puree without much pressure. If large chunks remain, continue cooking for another 15 to 20 minutes before mashing again.

Set a fine mesh sieve or food mill over a large bowl and carefully pour the pear mixture through it, pressing gently with the back of a spoon to push the soft solids through and leave behind any tough fiber or skin pieces. You should end up with about 4 to 5 cups of smooth puree. Discard the fibrous bits left in the sieve.

Pour the strained pear puree back into the cleaned pot and return it to low heat. Cook down the moisture for another 45 minutes to 1 hour, stirring frequently, especially toward the end, to prevent scorching on the bottom. The puree is done when it's noticeably thicker, clings to a spoon, and doesn't run off immediately when you tilt the spoon.

Drop a small spoonful onto a cold plate and let it cool for a minute; it should wrinkle slightly when you push it with your finger if it's ready. If it's still too loose, cook for another 10 to 15 minutes and test again. Remove from heat, stir in vanilla extract if you're using it instead of the bean, and let the butter cool in the pot for about 30 minutes before jarring or storing.