Preheat the oven to 150°C/300°F/Gas Mark 2. Line two baking trays with baking parchment.
1. Place the icing sugar and egg whites in a standing food mixer and whisk on high for 10 minutes until glossy white peaks form. Using a spatula, gently fold in the cornflour and the white wine vinegar. When this is mixed through, sift the cocoa powder and espresso powder over the meringue mix and gently fold in.
2. Using a tablespoon, place 3-inch/8cm spoonfuls of the meringue mix onto the baking tray and flatten each one gently in the centre with the back of a spoon. Bake for 45 minutes.
3. Place the sugar and water in a small saucepan over a medium heat and stir gently to dissolve the sugar. Add the espresso powder, whisk through and bring the mix to a steady simmer for 3 minutes. Remove from the heat, allow to sit for 1-2 minutes, then add the chocolate and whisk through until combined. Sift in the cocoa powder and whisk until smooth.
4. For the whiskey cream, whisk the cream into soft peaks. Dissolve the icing sugar with the whiskey in a bowl and then add to the cream, folding to combine. To assemble the meringues, place one on each plate, add a spoonful of whiskey cream on top, drizzle with the chocolate sauce and sprinkle with the toasted hazelnuts.