Whisk the double cream in a bowl. I like to do this by hand as I have complete control over it and I beat the cream until it just starts to feel heavy on my whisk - but don't over-whisk it.
Break the meringues into the cream - rough chunks are good. Scatter in the blackberries (they work perfectly with the meringue as they cut through the sweetness). To break up the smoothness of this sundae I add some pistachio nuts. Gently fold together so that the fruits burst through the cream.
Place a large scoop of vanilla ice cream in the bottom of two sundae glasses and top with spoonfuls of the blackberry cream and dust with icing sugar. This dessert has something soft, something crisp, something sweet and something sharp, which makes it perfection.