Line a large baking sheet with parchment paper or a silicone baking mat. Have all your ingredients measured and ready—this recipe moves fast once you start!
In a heavy-bottomed saucepan, combine the allulose, water, butter, and salt. Cook over medium-high heat, stirring constantly, until the butter melts and the mixture comes to a boil.
Attach a candy thermometer to the side of the pan. Add the peanuts and continue cooking, stirring frequently, until the mixture reaches 300°F (hard crack stage). This will take about 10-12 minutes.
Remove from heat immediately and quickly stir in the vanilla extract and baking soda. The mixture will foam up—this is normal and helps make the brittle light and crispy!
Pour the mixture onto your prepared baking sheet and use a heat-resistant spatula to spread it as thin as possible. Work quickly before it sets!
Let the brittle cool completely at room temperature (about 30-45 minutes), then break it into pieces. Store in an airtight container for up to 2 weeks.