Prep the pan: Lightly grease a rimmed baking sheet or line it with a silicone mat. Keep your baking soda, vanilla, butter, and peanuts measured and ready. Once the syrup hits temperature, you’ll need to move fast.

Combine sweetener and water: In a medium heavy-bottomed saucepan, add allulose and water. Stir gently just to moisten. If using sugar-free syrup, add it now.

Cook the syrup: Set over medium heat. Do not stir once it starts simmering. Clip on a candy thermometer and let it cook until it reaches 300°F (hard crack). This usually takes 8–12 minutes. If the edges darken too quickly, lower the heat.

Add butter and peanuts: Remove from heat and quickly stir in the butter until melted, then add peanuts. Stir to coat. Return to low heat for 15–30 seconds if needed to keep it fluid, but don’t overcook.

Flavor and aerate: Take the pan off the heat. Stir in vanilla and salt. Sprinkle baking soda over the mixture and stir briskly. It will foam and lighten in color—that’s what produces the brittle’s airy crunch.

Spread it out: Immediately pour onto the prepared baking sheet. Use a greased spatula to spread into a thin, even layer, about 1/4 inch thick. Work quickly before it sets.

Cool and break: Let cool completely, 20–30 minutes. Once firm, break into shards. Taste and adjust with a sprinkle of flaky salt if you like.