1. Spread half the nuts into a rectangle about 20 x 25cm on an ungreased baking sheet.
2. In a medium, heavy saucepan fitted with a sugar thermometer, heat the water, butter, salt, and white and brown sugars. Cook, stirring gently if needed, until the thermometer reads 150°C. Have the vanilla and baking soda ready.
3. Take off the heat and stir in the baking soda and vanilla.
4. Pour straight over the nuts and spread with a small offset spatula. Sprinkle with a little flaky sea salt.
5. Scatter the chocolate over the top and leave for 2 minutes, then spread it into an even layer.
6. Sprinkle over the remaining nuts and press them in gently. Cool completely and break into pieces. Keeps in an airtight container for up to ten days.