Berry Cloud

4 cups of blackberries (or use blueberries or strawberries) (about 3 pint containers)
1 1/2 cups plus 2 Tbsp. granulated sugar
Cooking spray
8 large egg whites, at room temperature
2 Tbsp. cornstarch
1 Tbsp. white wine vinegar
1 tsp. vanilla
1 1/4 cups cold heavy cream
1/4 cup powdered sugar, plus more for dusting
1/2 cup shelled salted pistachios, chopped

Wash and puree 1 1/3 cups of the berries in a food processor.  Strain through a fine sieve and discard solids.  Wash remaining berries and toss with 2 tablespoons granulated sugar.  Let stand at least 2 hours until berries release  juices.  Refrigerate until ready to use.  Can be done the day ahead.

Preheat oven to 325 degrees.  Line a 10 x 15 inch rimmed baking sheet with parchment paper and coat paper with cooking spray.  Whisk egg whites with a mixer until soft peaks form, 2 to 3 minutes.   Sprinkle with 1 1/2 cups granulated sugar.  Raise speed to high and whisk until stiff peaks form.  Whisk in cornstarch, vinegar, and vanilla.  Spread meringue evenly onto parchment paper in the sheet pan.  Bake, rotating halfway through, until puffed and top is pale gold, 20 to 22 minutes.  Let cool on sheet pan on a wire rack.

Whisk cream and confectioners sugar until soft peaks form.  Add 1/3 cup berry puree, and whisk until stiff peaks form. 

Run a knife around the edge of the meringue to release.  Generously dust top with confectioners sugar and top with another piece of parchment.  Flip pan, and peel off the original parchment.  Working with shorter end of the meringue facing you, spread berry cream on the cake, leaving a 1-inch border at the bottom and a 2-inch border at the top.  Tightly roll the cake into a 10 inch long log, starting with the bottom edge and using the parchment as a guide.  Refrigerate for at least 1 hour and up to 4 hours.  Trim the edges of the cake and slice into approximately1-inch slices.  Serve topped with reserved berries and sprinkled with pistachios.