Preheat the oven to gas 1, 140°C, fan 120°C. Line a baking tray with baking paper. Mix the caster sugar and brown sugar in a bowl. If there are any lumps in the brown sugar, rub them between your fingertips to break them up.
In a bowl, whisk the egg whites to soft peaks with an electric whisk. Continue whisking, adding the sugar 1 tablespoon at a time, until smooth and stiff, this should take about 5-6 minutes. Whisk in the cornflour and gently fold through 3/4 of the hazelnuts. Sift over the cocoa powder, then fold through 2 or 3 times until just marbled.
Divide the mixture into 6 mounds on the prepared baking tray. Use a dessert spoon to make a small indent in the top of each one. Scatter the remaining nuts over the meringues. Bake for 2 hours, or until the meringues lift easily from the baking paper. Turn off the oven, leave the door slightly open and allow to cool inside for 1 hour, then transfer to a wire rack.
When ready to serve, make the chocolate sauce. Put the hazelnut chocolate spread in a heatproof bowl with 2 tablespoons of just-boiled water and mix until smooth. Place the meringues on serving plates and top each with a scoop of ice cream and a scattering of pomegranate seeds. Decorate with mint sprigs, if using, then drizzle with the warm chocolate sauce to serve.
Top tip: make the meringues up to 3 days ahead and store in an airtight container at room temperature.
