Tape a 24- by 12-inch sheet of parchment paper on your work surface.
In a medium saucepan, bring the sugar, water and fine sea salt to a boil, stirring until the sugar is dissolved. Cook the mixture, without stirring, washing down any sugar crystals from the side of the pan with a pastry brush dipped in cold water, until the syrup registers 238ºF (soft-ball stage) on a candy thermometer, 10 to 12 minutes (sugar syrup will be colorless).
Remove the mixture from the heat and stir in 3/4 cup of the seeds with a wooden spoon, then continue stirring until syrup crystallizes, 3 to 4 minutes. The mixture will become dry and crumbly.
Return the pan to medium heat and cook, stirring constantly, until the sugar melts completely and turns a deep caramel color, 4 to 5 minutes.
Pour hot mixture onto parchment and cover with a second sheet of parchment paper. Immediately roll out the mixture between the two sheets as thin as possible with a rolling pin (pressing firmly).
Peel back the top layer of parchment and immediately cut the brittle into pieces with a sharp knife (or allow it to cool until firm and then break it into pieces). Cool brittle completely.
Melt the chocolate in a double-boiler or the microwave. Chop the remaining 1/4 cup pumpkin seeds. Dip the cooled brittle in the chocolate then immediately sprinkle the chocolate-covered area with the chopped pumpkin seeds and large-flake sea salt. Let cool until chocolate is firm then serve.