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1
Heat the oven to 325 degrees F and arrange a rack in the middle.
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2
Place the cookies in a food processor fitted with a blade attachment and process until very fine, about 30 seconds (you should have about 1 3/4 cups).
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3
Stop the motor, add the melted butter, and pulse until combined, about 5 (1-second) pulses.
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4
(Alternatively, place the cookies in a resealable plastic bag, press out the air, and seal.
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5
Using a rolling pin, smash the cookies into fine uniform crumbs, then transfer to a medium bowl, add the melted butter, and mix until evenly combined.)
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6
Pour the crumb mixture into a 9-inch pie plate and, using the bottom of a measuring cup or your fingers, press firmly and evenly into the bottom and up the sides.
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7
Bake until fragrant and slightly darkened in color, about 10 minutes.
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8
Remove the pie plate to a wire rack and set aside to cool.
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9
Meanwhile, make the chiffon filling.
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10
Place the water in a small bowl and sprinkle the gelatin evenly over the surface; set aside.
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11
Place the blackberries, 1/4 cup of the sugar, the zest, juice, and salt in a medium saucepan and stir to coat the blackberries in sugar.
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12
Mash the berries completely with a potato masher to release their juices, then bring to a rapid simmer over medium-high heat.
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13
Reduce the heat to medium low and simmer, stirring occasionally, until the berries are soft and falling apart and the liquid has thickened slightly, about 10 minutes.
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14
Meanwhile, place a fine-mesh strainer over a large bowl; set aside.
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15
Pour the blackberry mixture through the strainer.
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16
Using a rubber spatula, push on the blackberry solids and scrape the underside of the strainer until all of the liquid has been extracted; discard the contents of the strainer.
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17
Immediately whisk the gelatin mixture into the blackberry mixture until dissolved and smooth.
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18
Set aside to cool to room temperature, about 20 minutes.
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19
Place the egg whites in the bowl of a stand mixer fitted with a whisk attachment.
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20
Whisk on high speed until medium peaks form, about 1 minute.
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21
With the mixer still on high speed, slowly add the remaining 1/4 cup of sugar and continue whisking until stiff, glossy peaks form, about 30 seconds total.
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22
Rewhisk the blackberry-gelatin mixture, add half of the beaten egg whites to the bowl, and whisk until just combined.
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23
Add the remaining egg whites and gently stir with a rubber spatula until just combined and no streaks or lumps of egg white remain.
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24
Transfer the chiffon filling to the prepared crust and spread it into an even layer.
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25
Refrigerate uncovered until set, at least 1 hour.
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26
(If refrigerating overnight, cover lightly with plastic wrap once the chiffon filling is set.)
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27
When ready to serve, chill the bowl of a stand mixer and the whisk attachment in the freezer for 10 minutes.
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28
Add the cream and sugar to the bowl and whisk on high speed until medium peaks form, about 1 to 2 minutes.
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29
(Alternatively, you can use a hand whisk and a large chilled bowl.
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30
Whisk until medium peaks form, about 3 to 4 minutes.)
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31
Spread the whipped cream evenly over the pie, arrange the blackberries stem-side down in a tight cluster in the center of the pie, and serve immediately.
