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Cut off top half of each artichoke and trim base so that artichokes sit flat. Using a pair of sharp kitchen shears, cut off pointed tips of each artichoke leaf.
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Set a steamer basket in a large pot. Fill pot with enough water so that it comes just below steamer basket. Bring to a boil over high heat. Place artichokes in steamer basket, working in batches if necessary, and reduce heat to medium-low and cover. Steam until artichokes are tender and inner leaves can be easily removed, 45 to 50 minutes, adding more water to pot, if necessary.
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Remove artichokes from steamer basket and let cool until easy to handle. Using tongs and a spoon, remove and discard inner leaves and choke so that only the outer three to four layers remain, creating a large, hollow opening. Transfer artichokes to a large baking sheet and let cool completely. Artichokes can be steamed and hollowed up to 1 day in advance; cover with plastic wrap and transfer to refrigerator until ready to use.
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Preheat oven to 400 degrees.
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Generously brush artichoke leaves and hollowed cavity with melted butter; set aside. In a medium bowl, mix together breadcrumbs, olive oil, lemon zest, 1 1/2 teaspoon salt, and 1/2 teaspoon pepper.
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Divide breadcrumb mixture among artichoke leaves by gently pulling leaves away from centers and spooning breadcrumbs into the base of each leaf, avoiding the hollowed-out center but making sure to fill the leaves at the base. Gently pull stuffed leaves outward to create a wide opening for the souffle; set aside.
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In a small saucepan, whisk together flour and 1 1/2 teaspoons salt. Gradually whisk in half-and-half and place over medium-high heat; cook, whisking constantly until thick and smooth, about 5 minutes.
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Transfer half-and-half mixture to a large bowl. Scatter butter over top; butter will melt and help prevent a skin from forming. Let cool until lukewarm. Stir in egg yolks and cheese and set aside.
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In the bowl of an electric mixer fitted with the whisk attachment, beat together egg whites and egg white powder until medium to stiff peaks form. Quickly but gently fold egg white mixture into bowl with egg yolk mixture. Immediately divide evenly among hollowed artichoke centers and transfer to oven. Bake until souffle is puffed and golden brown, 45 to 50 minutes. Remove from oven and serve immediately.
