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1
Wash rhubarb and trim as little off the ends as possible. Cut rhubarb into 1-inch chunks. In a small saucepan, heat rhubarb, 1/4 sugar, and water on medium. Cook until rhubarb falls apart and there are no whole pieces left, adding water by the tablespoon if rhubarb sticks to the bottom of the pan. At this point, either use an immersion blender to puree the mixture, or (if you're like me and your blender is otherwise occupied) push the mixture through a strainer. The first method is definitely easier.
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2
Preheat oven to 350. Now, make the shortbread: blend all ingredients in a stand mixer or food processor until combined. Wrap in plastic and refrigerate about 1 hour. Then, either roll into 8x8 square and place square in 8-inch square baking pan, or dump dough into pan and use fingertips to press evenly into pan. Bake 30 minutes, until golden. Let cool on a rack or on the counter.
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3
Add a couple inches of water to the pot of a double boiler and set over medium heat. Put egg yolks, butter, remaining sugar, lemon zest, and lemon juice in the boil of the double boiler and whisk to combine. When sugar has dissolved completely, add the rhubarb puree by the spoonful, to temper the eggs. When all rhubarb has been added, set bowl over pot; the water should be simmering. Continue stirring the rhubarb mixture; after about 5 minutes, the mixture will be warm and slightly thickened. At this point, remove from heat. Press through a strainer -- this will give your curd that smooth, pudding-like texture.
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4
Use an offset spatula to spread curd evenly over shortbread. If you haven't eaten half the bowl right then and there, you should have enough curd to make a layer about the thickness of the shortbread; I didn't. Bake another 10 minutes, then remove from oven and cool on rack. Refrigerate about 20 minutes, and you'll find that they've firmed up enough to slice cleanly. Cut into 16 equal bars. Dust with powdered sugar before serving; do your best not to polish them all off in one sitting.
