Bring the blackberries, sugar and water to the boil in a medium pot and then simmer for 20 minutes. Let the syrup cool, then strain into a sterilised container, pressing on the fruit with the back of a spoon. The syrup will keep in the fridge for several weeks if you don't use it all.
Preheat the oven to 125C | 250F. Line a large tray with baking paper.
Put the egg whites and cream of tartar in the bowl of your stand mixer (making sure it is spotlessly clean or the meringue will flop). Lightly whisk together until frothy.
Start whisking at high speed and gradually add the sugar 1 tablespoon at a time.
Once all the sugar has been added, continue whisking at high speed until the meringue is glossy, forms stiff peaks and doesn't feel gritty.
Add the blackberry syrup and gently ripple it through the meringue.
Spoon large dollops of the meringue on the prepared tray, spaced slightly apart.
Lower the oven to 100C | 210F and bake the meringues for at least an hour or until they feel firm and their bases don't stick to the baking paper.
Turn the oven off, open the door slightly and leave the meringues in the oven until it is completely cool. Gently peel the meringues off the baking paper.
Whisk the cream and icing sugar until you have soft peaks. Add the rum and briefly whisk it in.
Top the meringues with the whipped cream, drizzle with some syrup and top with a few fresh blackberries.
The syrup and meringues can be prepared in advance. Omit the rum if serving to children – these are pretty potent! Vary the berries to whatever is seasonal – both raspberries and blueberries work well. The syrup can also be used in soft drinks and cocktails.