Prepare a baking sheet and line it with parchment paper or lighly butter it.
Prepare the baking soda, vanilla and butter so you are ready when it's the time to use them.
In a medium sized saucepan over medium high heat, bring the water, glucose, sugar, salt, and macadamia to a boil, stirring constantly with a wooden spoon or heatproof spatula. The macadamia will roast in the process.
Cover with a lid for a minute to dissolve any sugar crystals.
Remove the lid and cook, stirring occasionally, until it reaches 147 °C (296 degrees F). You have to use a thermometer here, making sure it doesn't touch the bottom of the pan.
Remove from the heat, add in the prepared vanilla, butter and baking soda and stir for a few seconds, until everything is well combined. The mixture will puff up.
Then immediately pour the brittle onto the prepared pan. Strech it with two forks, gently pulling in opposite directions making sure the macadamia nuts are not over each other.
Only start making the ice cream when the brittle is cooled completely, about 1 hour.