1. Simmer rhubarb with water and salt in small nonstick skillet until moisture is absorbed and rhubarb turns to mush.
2. Add brown sugar and continue cooking for 2 minutes until it becomes a paste. Remove from heat and transfer to a small stainless steel bowl.
3. Add herbs and refrigerate for 10 minutes.
4. Whip Plugrá Butter until smooth and light.
5. Add rhubarb mixture to butter and mix until well combined.
6. Use immediately, or transfer to an airtight container and refrigerate for up to 1 week.