Add all ingredients into the slow cooker. (You can also cook this recipe using the Instant Pot or stove; see Note 1.)
Cover and leave on HIGH for 1 hour.
Transfer contents to a blender and blend until very smooth. (I use my Blendtec on Smoothie mode, which is perfect for this.)
Pour blended mixture back into slow cooker.
Leave on LOW for 6 hours. Do not cover.
When desired consistency is reached, stir the mixture well before pouring into jar for storage.
Notes
Here are the instructions for Instant Pot or stovetop:
Instant Pot: Add all the ingredients to the Instant Pot. Use the Pressure Cook setting for 5 minutes on High mode, then quick release. Transfer to a blender and blend until smooth, then pour it back into the Instant Pot. Use the Slow Cook setting on Low mode for 6 hour with the lid off, stirring every half-hour.
Stove: Add all the ingredients to a heavy-bottomed pot, like a Dutch oven. Cover and bring to a boil, then reduce the heat to medium-low and simmer for 10 minutes. Transfer to a blender and blend until smooth, then pour it back into the pot. Using the lowest heat setting, cook for 3 hours, stirring every 15 minutes.
If you want to add spices: I recommend warm spices, like cinnamon, ginger, nutmeg, or cloves. Apple pie spice or pumpkin pie spice fit the bill as well. I wouldn't add more than 2 tsp of spices total. In fact, I like this rhubarb butter best on its own, with no additional flavourings. I love the smell and taste of rhubarb on its own. The spices change the flavour profile quite a lot and kinda obscure its fresh tartness.