put the rhubarb in a blender or food processor and grind it up, keep running it unit it gets juicy, you could add up to a TBSP of water to facilitate juicing and keep things moving

press the rhubarb in a sieve to get the juice out, you need a cup of juice

whisk the sugar and corn starch together in a saucepan and then whisk in the juice

set the pan over med/low heat and stir

gently whisk in the egg yolks and the butter until the butter is melted then switch to a wooden spoon

stir scraping the sides and the bottom of the pan over med/low heat until the curd is very thick and coats the back of the spoon

run it through the sieve to remove any lumps of egg or cornstarch

put it in a bowl and put plastic wrap right on the surface and cool in the fridge

once chilled it's ready to be used