- Measure the water, then sugar and corn syrup into a medium saucepan, and have the other ingredients measured and on hand, as well as a silicone mat or parchment-lined baking tray.
- Bring the sugar to a boil over high heat without stirring, occasionally brushing the sides of pan with water, and continue to boil until it turns a light amber colour, about 6 minutes.
- Remove the pan from the heat and whisk in the butter and baking soda (it will bubble up.) Quickly stir in the hazelnuts until they are coated and then immediately spread this onto the baking tray to cool and sprinkle with sea salt.
- Once cool, break off a few pieces and chop coarsely.
The brittle will keep for a week in an airtight container. Do not refrigerate.