Spread the macadamia nuts on a half sheet pan. Bake the nuts for 8 to 10 minutes, or until you begin to smell them and their color turns golden brown. Remove from the oven and allow to cool.
In a saucepan, melt the butter over medium heat. Cook, swirling the pan often, for 5 to 6 minutes, or until it starts to turn golden brown. Once it reaches a certain temperature it will darken quickly, so be sure to watch it — you don’t want it to burn. The boiling butter will sound like a water faucet running, and it’ll smell rich like caramel. Remove it from the heat and pour the butter into a separate dish to cool completely, leaving behind any burnt bits at the bottom of the pan.
Drain the soaked cashews, add them to a food processor or blender along with the cooled brown butter and pulse until a smooth paste is formed (this may take a few minutes).
Add the cashew butter paste to a large mixing bowl or stand mixer along with the brown sugar, vanilla and salt, and beat until combined. Add the flour and continue to mix until incorporated. Fold in the toasted macadamia nuts and the white chocolate chips, if using. Transfer the mixture to a container and refrigerate until chilled and ready to serve. Scoop into ice cream cones, if you like, and top with any of your other favorite toppings.