Fluffy whipped cream, soft meringue, and fresh blackberries cometogether for a rolled cloud cake that's lighter than air. This dessert gets its name from its billowy texture, but the nuts give it a satisfying, earthy crunch.
Toss 2 containers blackberries (about 3 cups) with 2 tablespoons granulated sugar; let stand for at least 2 hours, tossing occasionally, until berries release juices. Refrigerate until ready to use.
Meanwhile, preheat oven to 325 degrees. Line a 10-by-15-inch rimmed baking sheet with parchment; coat with cooking spray. Whisk egg whites with a mixer on medium speed until soft peaks form, 2 to 3 minutes. Sprinkle with remaining 1 1/2 cups granulated sugar. Raise speed to high, and whisk until stiff peaks form, about 5 minutes. Whisk in cornstarch, vinegar, and vanilla. Spread meringue evenly into sheet pan. Bake, rotating halfway through, until puffed and top is pale gold, 20 to 22 minutes. Let cool in sheet pan on a wire rack.
Puree remaining container blackberries (about 1 1/3 cups) in a food processor, and strain through a fine sieve; discard solids. Whisk cream and confectioners' sugar until soft peaks form. Add 1/3 cup puree, and whisk until stiff peaks form.
Run a knife along edge of cake to release. Generously dust top with confectioners' sugar, and top with another piece of parchment. Flip cake, and peel off original parchment. Working with shorter end of cake facing you, spread cream on cake, leaving a 1-inch border at bottom and a 2-inch border at top. Tightly roll cake into a 10-inch log, starting with bottom edge and using parchment as a guide. Refrigerate for at least 1 hour. Trim edges of cake, and slice. Serve topped with reserved berries and sprinkled with pistachios.
Make ahead: Berries can be refrigerated for up to 1 day. Storage: Rolled cake can be refrigerated for up to 4 hours.