Place the rhubarb, water, and 1/4 cup of the sugar in a saucepan and bring to a simmer over medium heat. Cook, stirring occasionally, until the rhubarb becomes tender and breaks down. Add more water, a tablespoon at a time, if the mixture starts sticking to the bottom of the pot. Remove rhubarb from heat. If you have an immmersion blender, use it to purée the mixture. If you don't have one, press the rhubarb through a mesh strainer. Place in a bowl and set aside at room temperature while making the shortbread. Place 12 tablespoons of the butter in the bowl of a food processor. Add the powdered sugar, salt, flour, ginger, and cinnamon, and pulse until combined. Shape into a ball, wrap in plastic wrap, and refrigerate for one hour. Preheat oven to 350˚F. Remove the dough from the fridge and roll it out on a lightly floured surface into an 8"x8" square. Place the dough on a baking dish and bake for 30 minutes, until golden. Allow to cool on a wire rack. Fill a double boiler with a few inches of water and place over medium heat. Place the egg yolks, reaming butter, remaining sugar, lemon zest, and lemon juice in the bowl of a double boiler and combine with a whisk. Heat through until the sugar has dissolved completely and remove from heat. Add the rhubarb purée to the sugar and lemon mixture, a spoonful at a time, then return to medium heat. Stir constantly for 5 minutes. It will have a smooth, pudding-like texture. Spread the rhubarb curd evenly over the shortbread, and place in the oven at 350˚F for 10 minutes. Remove from the oven and transfer to the refrigerator for at least 20 minutes. Slice into bars, dust with powdered sugar, and serve.