1. Line a baking sheet with parchment paper.
2. Combine first 3 ingredients in a medium, heavy saucepan over medium-high heat, stirring just until combined; bring to a boil. Cook, without stirring, until a candy thermometer registers 335° or until syrup is dark golden brown. Remove from heat; stir in pistachios and remaining ingredients (mixture will bubble). Quickly pour mixture onto prepared pan; spread to about 1/2-inch thickness. Cool completely (about 2 hours). Break brittle into bite-sized pieces.
Make-ahead tip: Cool the brittle completely, break into chunks, and store them in an airtight container at room temperature for up to one week.