Rhubarb Shortbread Bars from The Food Charlatan. Rhubarb Shortbread Bars are a dream come true! A thick and buttery shortbread crust, topped with a layer of simple rhubarb filling, and topped with even more glorious shortbread. There is a secret ingredient in this recipe that makes these rhubarb bars bright ruby red, and it’s not food coloring. It works like a charm! #easy #recipe