Puree enough peaches to fill your slow cooker at least 3/4 of the way up and pour them in.
Scrape a vanilla bean and add the seeds to the puree.
Set the cooker to low.
Put a wooden spoon or chopstick across the mouth of the cooker and set the lid on top of it. This way, you vent the cooking butter.
Cook for 2-3 hours and check. Stir and replace the propped lid.
Keep checking and stirring every hour or two.
If you need to go to bed, turn the butter off and set the lid on the cooker all the way. In the morning, prop the lid again and keep cooking.
When the butter seems quite thick and spreadable, taste it and sweeten it to taste.
Add some lemon juice at this time if you feel it could use a little brightening.
Using an immersion blender, puree the butter so that it is smooth and emulsified.
Funnel the butter into clean, hot half pint jars, leaving 1/2 inch headspace. Wipe the rims, apply lids and rings, and process in a boiling water bath canner for 15 minutes.
Eat on toast, stirring into yogurt, or baked into quick breads all winter long.