Blackberry Chiffon Pie

Heat the oven to 325°F and arrange a rack in the middle.

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). Stop the motor, add the melted butter, and pulse until combined, about 5 (1-second) pulses. (Alternatively, place the cookies in a resealable plastic bag, press out the air, and seal. 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.)

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.

Bake until fragrant and slightly darkened in color, about 10 minutes.

Place the water in a small bowl and sprinkle the gelatin evenly over the surface; set aside.

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. Mash the berries completely with a potato masher to release their juices, then bring to a rapid simmer over medium-high heat. 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. Meanwhile, place a fine-mesh strainer over a large bowl; set aside.

Pour the blackberry mixture through the strainer. 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. Immediately whisk the gelatin mixture into the blackberry mixture until dissolved and smooth. Set aside to cool to room temperature, about 20 minutes.

Place the egg whites in the bowl of a stand mixer fitted with a whisk attachment.

Whisk on high speed until medium peaks form, about 1 minute. 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.Rewhisk the blackberry-gelatin mixture, add half of the beaten egg whites to the bowl, and whisk until just combined.

Add the remaining egg whites and gently stir with a rubber spatula until just combined and no streaks or lumps of egg white remain.

Transfer the chiffon filling to the prepared crust and spread it into an even layer. Refrigerate uncovered until set, at least 1 hour. (If refrigerating overnight, cover lightly with plastic wrap once the chiffon filling is set.)To assemble:When ready to serve, chill the bowl of a stand mixer and the whisk attachment in the freezer for 10 minutes.

Add the cream and sugar to the bowl and whisk on high speed until medium peaks form, about 1 to 2 minutes. (Alternatively, you can use a hand whisk and a large chilled bowl.

Whisk until medium peaks form, about 3 to 4 minutes.)

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.