Place all ingredients in a heavy saucepan and stir to combine.

Bring to a boil and then, quickly reduce to a simmer to minimize sputtering.

Simmer for 20 to 30 minutes, stirring often and wiping the sides of the pan with a spatula to reduce buildup.

When the sweet potato mixture is thickened and darkened slightly, remove from heat and pour into sterilized jars. (Jars and their lids can be run through a quick cycle in the dishwasher to sterilize while the butter simmers.)

Pour the hot potato mixture into hot jars, leaving ¼ inch headspace before sealing with lids and rings. Store for two to three weeks in the fridge, or use a home canner to make the butter safe for room temperature storage.