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Prepare the crust: Sift flour, baking powder, salt, and sugar together. Using a pastry blender or fork, cut in the butter until broken into pea-sized pieces. In a separate small bowl, beat the egg and milk together. Add the mixture to the dry ingredients and stir until combined. Press the dough into the bottom of a greased 13x9 inch cake pan.
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Prepare the filling: In a medium mixing bowl, mix the sugar, flour, and salt. Add the eggs, milk, and vanilla, and whisk together until thoroughly mixed. Add the rhubarb and stir. Pour the mixture over the crust.
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To bake: Bake at 350 degrees for 45 minutes, or until custard has set and is a light golden brown.