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1
Combine the berries and sugar in a medium bowl.
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2
Let sit at room temperature, stirring occasionally, while you make the cake.
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3
Heat the oven to 350 degrees F and arrange a rack in the middle.
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4
Cut 2 (18-inch) pieces of parchment paper.
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5
Trim 1 of the pieces to 18 by 13 inches; set both aside.
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6
Coat an 18-by-13-inch rimmed baking sheet with butter, line it with the trimmed piece of parchment, and coat the top of the parchment with more butter; set aside.
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7
Place the egg yolks, 1/3 cup of the sugar, and the vanilla in the bowl of a stand mixer fitted with a paddle attachment.
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8
Beat on high speed until pale yellow and thickened, about 5 minutes.
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9
Stop the mixer and scrape down the sides of the bowl and the paddle.
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10
With the mixer on low speed, slowly add the flour and mix until just combined, about 1 minute total.
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11
Transfer the mixture to a large bowl and set aside.
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12
Thoroughly wash and dry the mixer bowl, return it to the mixer, and fit the mixer with a whisk attachment.
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13
Place the egg whites, cream of tartar, and salt in the bowl and whisk on high speed until soft peaks form (they should droop over like soft-serve ice cream), about 1 minute.
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14
Gradually add the remaining 1/3 cup of sugar and continue whisking until stiff peaks form (standing straight up), about 1 minute more.
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15
Using a rubber spatula, fold a quarter of the egg whites into the reserved yolk-flour mixture until evenly incorporated; gently fold in the remaining egg whites until just combined and no large white streaks remain.
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16
Pour the batter into the prepared pan.
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17
Using an offset spatula, spread it into a very even layer, smoothing the top.
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18
(For the best results, make sure the surface of the cake is as smooth and even as possible.)
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19
Bake until the cake is starting to pull away from the sides of the baking sheet, the top springs back when pressed with your finger, and its no longer sticky to the touch, about 20 minutes.
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20
Meanwhile, place a large, clean metal or glass bowl and a clean whisk in the freezer to chill for the whipped cream.
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21
Remove the cake from the baking sheet by placing the second piece of parchment over top of it.
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22
Place an upside-down wire rack over the parchment.
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23
Flip the baking sheet over (be careful while handling the hot pan) and gently peel off the bottom piece of parchment if its sticking; discard the parchment.
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24
Allow the cake to cool on the parchment-covered rack while you make the whipped cream.
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25
Place the cream, sugar, and vanilla in the chilled bowl and whisk with the chilled whisk until medium peaks form, about 3 minutes.
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26
Place the bowl in the refrigerator.
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27
Transfer the cooled cake with the parchment to a cutting board.
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28
Using a serrated knife, trim the edges of the cake to make a 16-by-11-inch rectangle.
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29
Cut the longer side of the rectangle into 4 (4-inch) strips.
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30
Cut the shorter side into 4 (2-3/4-inch) strips.
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31
You should have 16 (4-by-2-3/4-inch) rectangles.
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32
Prepare 8 small serving plates.
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33
Using a flat spatula, place 1 cake portion on each plate.
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34
Divide half of the reserved strawberries over the cakes, and half of the whipped cream over the berries.
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35
Top with another cake portion and repeat with the remaining berries and whipped cream.
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36
Serve immediately.
