Combine rhubarb, sugar and water in a medium saucepan over medium-high heat. Bring to a boil, then reduce heat to medium and cook, stirring frequently, until rhubarb breaks down to a sauce - about 15 minutes. Stir in lemon juice. Remove from heat and cool 5 minutes.

Meanwhile, whisk eggs and yolks together in a medium bowl. Very slowly, about a tablespoonful at a time, whisk 1 cup of the rhubarb sauce into the eggs, then gradually whisk the egg-rhubarb mixture back into the saucepan with the remaining rhubarb.  Cook over medium-low heat, stirring constantly, for about 7 minutes, until it noticeably thickens (or reaches 165ºF on a thermometer).  Stir in butter.  Cool completely before serving.  Curd will keep for about 5 days in the fridge.