Preheat the oven to 250 degrees F. Line a baking sheet with parchment paper. Draw a heart on the paper that is 7 inches wide, at its widest part, and 4 inches tall in the centre. Beat the egg whites and cream of tartar with an electric mixer until very soft peaks form. Gradually add the berry sugar, beating constantly, until all is incorporated. Keep beating until the mixture is thick and glossy and stiff peaks form, and then beat in the cornstarch and vanilla. Spoon and spread the meringue inside the borders of the heart you’ve drawn on the paper. Build up the sides of the heart, creating shallower areas of meringue in the middle of the heart to later fill with whipped cream and fruit.
Bake 90 minutes, and then turn off the heat and leave the meringue in the oven to cool and dry for 2 hours. (The meringue can be made up to a day in advance. Store in an airtight container until ready to serve.) Whip the cream until stiff peaks form and then spread on top of the meringue.
Decorate the top of the pavlova with the fruit. Dust with icing sugar and serve.
Note: Berry sugar is very finely granulated sugar that’s sold in small bags. It can be found in the aisle where other sugar products are sold.