Preheat the oven to 170C/338F. Place the nuts on a baking sheet and into the oven. The aim is to roast them, but also to keep them hot. Cold nuts will cause the brittle to seize up if you add them to the hot caramel, which is why we want them to be warm. If your nuts are already roasted or getting too brown, turn the oven right down or cover them with foil.
Place the butter in a dutch oven or large heavy bottomed pan over a low, even heat. Cook the butter until it is golden, nutty and brown, 3-5 minutes.
Add the remaining ingredients and stir to combine. The mixture should gradually begin to bubble up and keep an even keel of bubbles for the remainder of the cooking time. Stir intermittently using a spatula to keep the sugar moving.
Cook the mixture for 10-15 minutes or until a thermometer reaches 149C/300F. The mixture should be caramel in colour and not darkened. Watch it very carefully in the last 5-7 minutes of cooking, as it can progress from undercooked to burnt very quickly. It should absolutely not smell burnt - get it off the minute you smell burnt sugar.
Sprinkle over the baking soda and stir to combine. The mixture should begin to bubble up like honeycomb. Add the hot nuts and stir to combine, before quickly (and VERY carefully) pouring the mixture onto a baking sheet. I find there's no need to grease or line the tray as the caramel sets and can be pulled away from the tray, but you can if you prefer.
Allow the brittle to harden and set for up to 1 hour until it snaps easily. Store in an airtight container on the bench.