Preheat oven to 250 degrees Fahrenheit.
Line a baking sheet with parchment paper.
Beat egg whites and cream of tartar with a stand mixer until soft peaks form.
Gradually add sugar, one tablespoon at a time, while whipping on high speed until the mixture is glossy and holds stiff peaks.
Gently fold in the sifted cornstarch.
Spoon the meringue onto the baking sheet, shaping into eight individual nests.
Bake for 90 minutes and allow to cool completely in the oven with the door closed.
Simmer blackberries in a pan until reduced into a compote.
Whip the heavy cream until stiff.
Top cooled meringue nests with whipped cream and the blackberry compote.