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1
Preheat the oven to 350 degrees Fahrenheit. Grease a 13x18 inch baking sheet (half sheet pan) generously with nonstick spray.
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2
Make the biscuit: in a food processor, pulse the flour, brown sugar, baking powder, baking soda, and salt. Add the butter and pulse until they are the size of peas or smaller. Transfer the mixture to a large bowl. (Note: you can also do this by hand, but it will take a bit longer.)
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3
In a medium bowl, whisk together the buttermilk, eggs, egg yolk, and vanilla together to combine. Add this mixture to the flour mixture and mix well to combine. The mixture should be uniform - no dry flour patches - I use my hands to make sure it's combined - try not to overmix.
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4
Transfer the dough to the prepared pan and press into an even layer (you can lightly moisten your hands if necessary). In a small bowl, whisk the egg and cream together to combine. Brush the mixture over the biscuit (you should use the whole thing), then sprinkle generously with turbinado sugar.
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5
There's two ways you can bake the biscuit, depending on your preferences. For a lighter, fluffier, cakier biscuit, bake on the middle shelf of the oven until the biscuit is evenly golden brown on top and when pressed in the center it seems set, 30-35 minutes. For a crispier, more biscuit-like biscuit, extend baking time until the biscuit is very brown on top, 5-10 minutes more.
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6
In a large bowl, toss the rhubarb, sugar, lemon zest, lemon juice, and vanilla bean to combine. Spread the rhubarb onto a baking sheet and roast in the lower third of the oven until just tender, 25-30 minutes.
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7
Let the biscuit and the rhubarb cool completely.
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8
In the bowl of an electric mixer fitted with the whip attachment, mix the cream cheese and powdered sugar until light and fluffy, 3-4 minutes. Add the cream gradually and until very thick and smooth (medium peaks), 4-5 minutes. Mix in the vanilla.
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9
You can make all three components of the shortcake up to 1 day ahead, but it's best to assemble just before serving. Once assembled, it will start to get a little soggy within 1-2 hours.
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10
To assemble, spread the whipped cream onto the biscuit in an even layer. Spoon the rhubarb mixture and it's juices on top and serve.
