1-Generously coat a half-sheet pan (or two: Generously coat a half-sheet pan (or two quarter sheet pans) with butter.
2-Combine 2 cups (400g) sugar, ½ cup: Combine 2 cups (400g) sugar, ½ cup (165g) light corn syrup, ½ cup (165g) dark corn syrup, ½ cup water and ¼ teaspoon salt in a saucepan. Cook over medium heat, stirring, until sugar has dissolved, 3-5 minutes.
3-Bring mixture to a boil (this will: Bring mixture to a boil (this will take 5-8 minutes), add 1 cup (226g) butter and cook, stirring frequently until mixture reaches 280ºF (138ºC) (soft crack stage), 15-20 minutes.
4-Stir in 3 cups (410g) peanuts and: Stir in 3 cups (410g) peanuts and continue to cook until mixture reaches 305ºF (151ºC) (hard crack stage), about 10 minutes.
5-Off heat, stir in 1 teaspoon vanilla: Off heat, stir in 1 teaspoon vanilla and 1 teaspoon baking soda and stir quickly (mixture will foam). Immediately pour mixture into prepared sheets, dividing evenly.
6-Using a greased spatula, press brittle into: Using a greased spatula, press brittle into an even layer.
7-Sprinkle brittle lightly with additional kosher salt: Sprinkle brittle lightly with additional kosher salt.
8-Let brittle cool completely then break into: Let brittle cool completely then break into pieces. Store in an airtight container at room temperature for up to 1 month.