Preheat oven to 350 F. In a medium bowl, blend together butter, 2 cups flour, and 1/2 cup sugar until mixture is crumbly. Press firmly and evenly into the bottom of a 9 x 13-inch pan lined with parchment paper. Bake 15 to 20 minutes or until firm and lightly golden in color. Set aside to cool.
While the crust is cooking, start by making a rhubarb puree. Combine puree ingredients in a medium saucepan and cook on medium-high until rhubarb has turned into a sauce with very few chunks, about 10 to 15 minutes. Transfer to a blender or food processor and puree until smooth. Set aside to cool before moving on to the next step.
In a large bowl, whisk together curd ingredients except for red dye and flour. Slowly add red dye and whisk until incorporated. Stop when you reach the desired color (the color will fade a bit when cooked). Lastly, whisk in the flour until smooth. Pour curd on top of crust and bake for 10 to 15 minutes, or until set. The bars will continue to firm up as they cool. Chill in refrigerator for 4 hours. Pull the parchment paper up on all sides, lift, and lay on a flat surface for easy cutting of bars.
There will be more rhubarb puree than the recipe calls for. Spoon some on top of the bars when serving for extra tartness.
Per serving: 326 calories, 17.5 grams fat (10 grams saturated fat), 38 grams total carbohydrates, 1.5 grams fiber, 5 grams protein.
