Using a food processor, puree the rhubarb until smooth, about 2-3 minutes. If the mixture looks too dry, add a teaspoon or so of water to it to loosen it up, but don’t over do it.
Using a cheesecloth or a fine mesh sieve, squeeze out the rhubarb juice and discard remaining pulp. You should end up with about ⅔ cup of juice.
Add two inches of water to the bottom pot of a double boiler* and bring to a consistent simmer. In the top pot, add rhubarb juice, sugar, egg yolks, eggs, and salt. Place pot over simmering water and whisk constantly until liquid thickens and coats the back of a wooden spoon, about 18-20 minutes. *If you don’t have a double boiler, you can use a pot and a metal bowl that sits on top.
Remove pot from heat and add in butter. Whisk until butter melts completely. Let cool in pot before transferring curd to two 8-ounce canning jars (or equivalent). Keep in refrigerator for at least 4 hours before serving, allowing the curd to firm up.