Preheat your oven to 225°F (110°C). Line a baking sheet with parchment paper and set aside.
In a clean, dry mixing bowl, beat the egg whites on medium speed until frothy.
Add the cream of tartar and continue beating until soft peaks form.
Gradually add the granulated sugar, a tablespoon at a time, while continuing to beat the egg whites. Beat until stiff, glossy peaks form. This may take about 7-8 minutes.
Gently fold in the vanilla extract and chopped mixed berries until evenly distributed throughout the meringue mixture.
Spoon the meringue mixture into a piping bag fitted with a large star tip or simply use a spoon to dollop the mixture onto the prepared baking sheet, shaping it into heart shapes.
Bake the meringue hearts in the preheated oven for 1 hour, or until they are dry and crisp on the outside. The insides should remain soft and marshmallow-like.
Once baked, turn off the oven and leave the meringue hearts inside to cool completely with the oven door slightly ajar. This helps prevent them from cracking.
Once cooled, carefully remove the meringue hearts from the baking sheet and transfer them to a serving platter.
Dust the meringue hearts with powdered sugar just before serving for a decorative touch.
Enjoy these delightful berry meringue hearts as a light and airy dessert that's bursting with fruity flavor!