Preheat the oven to 300°F and line a baking sheet with a silicone baking mat or parchment paper.

Add the egg whites and cream of tartar to a large bowl. Use an electric mixer with a whisk attachment to beat the egg whites on medium-low speed until soft peaks form.

Once the egg whites can hold a soft peak, slowly add the caster sugar, 1 tablespoon at a time, continuing to mix the whole time, until all the sugar is added. When the meringue is ready, it should be very thick.

Divide the meringue into 6-8 mounds on the prepared pan, leaving some space between each mound, as they will spread when they bake. (Bake on two baking sheets, if needed.) Form each mound into a round nest with an indentation in the center.

Bake for 30 minutes, then turn off the oven and leave the meringues in the oven for 3 hours to dry out and cool slowly. Remove from the oven, cool to room temperature, and store in an airtight container until ready to serve.

Before serving, make the whipped cream by beating together the heavy cream, powdered sugar, and vanilla extract until soft peaks form.

To serve, fill each mini pavlova with whipped cream and top with fresh berries.