Pumpkin Seed Brittle

Put a 24- by 12-inch sheet of parchment on a work surface and anchor corners with pieces of tape. Bring sugar, water, and sea salt to a boil in a 2-quart heavy saucepan over moderate heat, stirring until sugar is dissolved. Cook mixture, without stirring, washing down any sugar crystals from side of pan with a pastry brush dipped in cold water, until syrup registers 238°F (soft-ball stage) on thermometer, 10 to 12 minutes (sugar syrup will be colorless). Remove from heat and stir in seeds with a wooden spoon, then continue stirring until syrup crystallizes, 3 to 4 minutes.
Return pan to moderate heat and cook, stirring constantly, until sugar melts completely (sugar will continue to dry and become grainy before melting) and turns a deep caramel color, 4 to 5 minutes more (seeds will be toasted). Carefully pour hot caramel mixture onto parchment and carefully cover with another sheet. Immediately roll out (between sheets of parchment) as thinly as possible with a rolling pin, pressing firmly. Remove top sheet of parchment and immediately cut brittle into pieces with a heavy knife or pizza wheel. Cool brittle completely, then peel paper from bottom. (Alternately, break brittle into pieces once cool.)