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1
Make the pastry cream, so it can chill while you work on the rest. In a medium pot, bring the milk, vanilla bean, and 1/4 cup of sugar to a simmer over medium heat.
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2
In a medium bowl, whisk the remaining 1/4 cup sugar and cornstarch together to combine. When the milk is nearly at a simmer, whisk the egg yolks into the sugar/cornstarch mixture.
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3
Gradually pour about 1/3 of the warm milk mixture into the yolk mixture, whisking constantly to combine. Return the mixture to the pot, whisking well to combine.
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4
Continue to cook over medium-low heat, stirring constantly (I usually switch to a silicone spatula at this point to make sure I can get into the corners/edges of the pot), until the mixture comes to first boil - a large bubble breaking the surface from the center of the pot, not just small bubbles around the edges.
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5
Strain the mixture into a shallow dish, and cover directly with plastic wrap. Refrigerate until fully chilled.
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6
Make the puff pastry layers: preheat the oven to 400u00b0F, and line two baking sheets with parchment paper. Divide your puff pastry in half, and roll one half out on a lightly floured surface into a large rectangle (about the size of the baking sheet) about 1/4 inch thick.
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7
Transfer the pastry to one of the parchment lined baking sheets and trim with a pastry wheel or knife so the edges are straight. Repeat with the second piece of puff pastry.
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8
Cut each of the sheets into 12 rectangles (about 2 x 4 inches - you can eyeball it by cutting the pastry in half horizontally, then into six even strips vertically). Dock the rectangles well with a fork.
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9
Place a piece of parchment paper on top of each pan of pastry rectangles. Place another baking sheet on top of the parchment - this helps weigh the pastry down during baking so it doesn't puff up too much.
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10
Bake the pastry layers for 15 minutes, then remove the top baking sheet and parchment and bake for 4-5 minutes more, until they are very golden brown. Let cool completely.
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11
In a mixer or by hand, whip the cream to medium peaks. Once the pastry cream is fully cooled, fold the whipped cream into the pastry cream gently to combine. Transfer the mixture to a pastry bag and snip a 1/4 inch opening from the end.
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12
In a medium bowl, toss the strawberries with the vanilla sugar.
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13
To assemble, place one layer of puff pastry on a plate or serving platter. Pipe dots of filling all over the pastry, then top with another layer. Repeat until there's three layers of pastry and three layers of filling, then arrange strawberries on top.
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14
Repeat with the remaining pastry, filling, and strawberries. If desired, dust powdered sugar over the finished mille feuille. Serve immediately, or refrigerate for up to 5 hours until ready to serve.
