Preheat the oven to 160ºC/gas 2-3. Line a Swiss roll tin (about 23 x 30cm) with a large sheet of baking paper.
With an electric hand whisk or a free-standing mixer, whisk the egg whites with a pinch of sea salt until stiff.
Add the sugar a little at a time and keep whisking until all of it is combined. Sift in the cornflour and whisk on high for 5 minutes, or until stiff and glossy.
Spoon onto the paper-lined tin and bake for 20 to 25 minutes, or until crisp on the outside and cooked through – it should be fluffy on the inside. Leave to cool.
Meanwhile, halve the vanilla pod and scrape out the seeds, then whisk with the cream and the elderflower until soft peaks form. Squish a handful of the berries and swirl them through the mixture.
Lay the cooled meringue on a fresh piece of baking paper (remove and discard the old one). Spread with a layer of rippled cream to about three-quarters of the way along, then top with a layer of berries.
Carefully roll the meringue, shortest end to shortest end, using the paper to help you. Serve on a board or platter.