In a saucepan, combine the rhubarb and water. Bring to a simmer over medium heat and cook for a few minutes, just until the rhubarb starts to soften.
Pour the mixture into a blender, and puree until smooth. Set a strainer over the saucepan, and pour the rhubarb puree through the strainer so you can collect the liquid, but discard the pulp.
Bring the liquid back to a simmer over medium heat, and boil until it's reduced to 1/2 cup. Set aside to cool to room temperature.
Now, set up a double boiler. You can do this by placing a saucepan with several inches of water on the stove, and bring it to a simmer over medium heat. You'll also need a non-reactive stainless steel bowl to set on top of the saucepan. The bottom of the bowl should not directly touch the water.
In your stainless steel bowl, whisk together the sugar, strawberry powder and corn starch. Whisk in the egg yolks and the reduced rhubarb liquid.
Set the bowl on top of the pan of simmering water. Cook the rhubarb curd over the water, stirring constantly but gently with the spatula – no need to stir aggressively. Continue stirring and cooking the rhubarb curd, until it's thickened enough to coat the back of a spoon, and it leaves tracks against the side of the bowl (this means when you swirl the curd against the side of the bowl, run your finger through the curd – if it's thick enough, your finger will leave a track through the curd without the curd running back through). This process can take anywhere from 15-20 minutes, depending on how vigorously your water is boiling.
Remove the bowl from the heat. Whisk in the cold butter, a few pieces at a time, until it's completely disappeared and the curd is smooth. If the rhubarb curd has cooled down too much and you're having trouble getting the last few pieces of butter to melt, you can place the bowl back over the hot water, long enough to finish whisking in the butter.
Now strain the curd (to remove any bits of cooked egg) by passing it through a wire mesh strainer. It will thicken more as it cools.
Store the rhubarb curd in a lidded glass jar, and refrigerate for up to 2 weeks.