Notes: If using frozen berries, thaw in a colander over a bowl for about 1 1/2 hours. In step 5, stir the cornstarch mixture into the berry juice to thicken, then fold in thawed berries. You can make meringues up to 1 day ahead (steps 1 through 4); cool completely and store airtight. You can prepare berry mixture (step 5) up to 4 hours ahead; cover and chill.
4 large egg whites (1/2 cup)
1/2 teaspoon cream of tartar
About 1 1/3 cups sugar
6 cups fresh or frozen blackberries (see notes)
2 teaspoons cornstarch
1 cup whipping cream
Fresh mint sprigs
How to Make It
Line two 14- by 17-inch baking sheets with cooking parchment or buttered and floured foil. Draw eight 4-inch circles on the parchment, spaced at least 3 inches apart.
In a bowl, with a mixer on high speed, beat egg whites with cream of tartar until foamy. Gradually add 1 cup sugar, beating until stiff peaks form.
Mound mixture equally on circles and form a slight depression in the center of each with the back of a spoon, or spoon the meringue into a pastry bag fitted with a 1/2-inch-diameter plain or star tip and pipe mixture to fill in circles completely.
Bake in a 250° oven (or 225° convection oven) until meringues begin to turn pale gold and feel firm to touch, about 1 1/4 hours (1 1/2 hours in convection oven); switch pan positions halfway through baking. Turn oven off and leave meringues in oven until cool, 2 to 2 1/2 hours.
Meanwhile, in a 2- to 3-quart pan, mash 1 cup berries. Mix cornstarch and 1/3 cup sugar; stir into mashed berries. Stir berry mixture over high heat until boiling. Remove from heat and let stand, stirring occasionally, until cool to touch, 5 to 10 minutes. Fold in remaining 5 cups berries. Taste, and add more sugar if desired.
Just before serving, in another bowl with a mixer on high speed, beat cream until soft peaks form. Add sugar to taste and beat until blended.
Set meringues on plates. Top equally with berry mixture and whipped cream. Garnish with mint.