1. Place all of the ingredients together in a small saucepan, and place it over medium-high heat. Once the mixture begins to bubble and boil, turn it down to medium-low and allow it to simmer, about 25 minutes, stirring every few minutes. 
2. Remove the pot from heat, then blend the mixture until smooth, using either an immersion blender, high-powered blender, or food processor. Pour the smooth mixture back into the pot, then place it back on the stove over low heat. 
3. Simmer the rhubarb butter for 2 hours, stirring every so often, until thickened. (It will thicken more as it cools). Pour the butter into a heat-proof container or jar, then place it in the fridge to cool completely, about 4-5 hours. Once cooled, you can use it as a spread on toast, pancakes, or in a baking recipe. 

Rhubarb butter will keep in the fridge, in an airtight container, for 10 days to 2 weeks, tightly sealed. Once the butter has completely cooled, you can freeze it for up to months. Thaw overnight in the fridge.