1lb. (454 g) fresh rhubarb, cut into ½-inch/1 cm pieces - about 4 cupsUse the reddest rhubarb stalks you can find, or just use the redder bottom half of each stalk.
¾cup(150 g) sugar*see note, below
apinch(~⅛ teaspoon) fine sea salt
1tablespoonfresh lemon juice
3 tablespoonswater
1tablespoonbutter
½teaspoonpure vanilla extract
Instructions
Combine the rhubarb with the sugar, salt, lemon juice, and water in a medium-sized heavy-bottomed saucepan. Bring to a boil, stirring often to prevent sticking. Then reduce the heat to medium, and continue to cook the rhubarb, uncovered, until it is soft and broken apart, stirring occasionally. This will take 15-20 minutes.
Scrape the rhubarb into the canister of a high-speed blender and purée until it is very smooth, scraping down the sides as necessary. Alternately, you can purée it in the saucepan with an immersion blender, but it might not be quite as silky-smooth (though still just as delicious).
Return the puréed rhubarb to the saucepan, and cook it, uncovered and stirring constantly, over medium heat until it is very thick - if you scrape a flat-bottomed wooden spoon or spatula through it, the channel doesn't fill back in. This will take another 10-15 minutes. Taste and adjust for sweetness, adding more sugar if desired, and cook for another minute.
Remove the rhubarb butter from the heat and stir in the butter and vanilla. Let cool.
Transfer the rhubarb butter to a sealable jar or container, and keep it refrigerated for up to a month. The rhubarb butter can also be divided into smaller containers and frozen for up to a year.
Makes about 1¾ cups (420 ml), depending on how much you cook it down.
Notes
The redder your rhubarb, the more intensely ruby your rhubarb butter will be. However, no worries if your rhubarb is more green than red - it will still taste the same.To get the reddest rhubarb butter, I like to use the bottom red half of each rhubarb stalk and save the greener top parts to use for other recipes.*Adjust the amount of sugar to your taste. Rhubarb varieties vary in tartness, and rhubarb can be tarter at the beginning of the season compared to the end of the season. You could start with ½-⅔ cup (100-135 g) of sugar, then add more to taste, a tablespoon at a time after you've cooked the rhubarb butter down. Just cook it for a minute or two after the last addition of sugar.