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.