Combine all the ingredients in your slow cooker and stir well to combine.
Vent the lid (I used a chopstick) to keep it slightly open.
Cook on low for 7-9 hours. The butter should reach an internal temperature of at least 135 degrees F. before it's done cooking for food safety. Simply check it with a thermometer before turning it off.
Transfer to a flat, casserole dish, spreading the butter out evenly. Cool in the fridge. Once cooked, spoon into storage containers.
This should be kept in the fridge, but it also freezes well if you feel you have too much to eat within a week or so. Do not freeze this in glass jars as you risk the glass breaking.
Please note that the nutrition data below is a ballpark figure. Exact data is not possible. Also, there is no way for me to know what your final yield will be, so I'm giving data for the entire batch. You'll have to measure your final yield and divide that into acceptable serving sizes to get the final data.