In a small narrow mixing bowl, using an electric mixer, beat the egg whites, cream of tartar, and salt on high until soft peaks form. Slowly add the sugar while beating on high until stiff peaks can be formed. Beat in the vanilla.
Divide the beaten egg whites into two equal-size mounds on a baking sheet lined with parchment paper or a silicone mat. Use the back of a spoon to shape the mounds into 3.5-inch-diameter round bowls.
Bake for 1 to 1 ¼ hours, or until the meringues are set and dry. Turn the oven off and leave the meringue bowls in the oven for 1 hour. Remove the meringues to cool completely on wire racks.
In a small bowl, using an electric mixer, whip the heavy cream and vanilla sweetener (if using) until soft peaks can be formed. When the meringue shells are cool, spread the whipped cream into each, then top with the berries.
Notes
Allowing egg whites to stand at room temperature for 30 minutes will result in a fuller volume after beating.The meringue shells can be made in advance and stored in an airtight container for 3 to 5 days in a cool, dry spot.Use a superfine sugar as it will dissolve more quickly than granular sugar.