Line a large cookie sheet with parchment paper or a silicone baking liner. Place in an oven heated to 200 degrees F until ready to use.

Place the sugar in a large, heavy saucepan over high heat, and stir with a wooden spoon until it begins to melt. Lower the heat to medium-high and keep stirring just until the sugar is melted. Stop stirring and watch for it to turn medium caramel, or about 250 degrees F on a candy thermometer. Stir in warm pecans (they need to be warm or the sugar could crystallize), then add the butter. Allow to cook for about 2 minutes, stirring constantly, or until thoroughly combined and the butter is incorporated; this will be 300 degrees F on a candy thermometer. Stir in half of the rosemary and half of the sea salt. Remove from heat.

Working quickly, grab your cookie sheet from the oven, turn the mixture out onto the pan, and spread it evenly to the desired thickness with a wooden spoon or a stiff rubber spatula. Sprinkle remaining rosemary and salt on top, while the brittle is still warm.

Allow to cool completely for at least 1 hour, then break the brittle into pieces and store in an airtight container at room temperature. If your brittle isn't brittle enough to break into pieces, pop it in the freezer for 10 to 15 minutes, until it hardens enough to snap easily.