In a deep, large saucepan (about 3 to 4 qt size) with candy thermometer attached, combine the sugar, butter, water, and corn syrup. Cook over medium-high heat, stirring frequently, until the mixture registers 300 F.

Remove the pan from the heat and immediately stir in the baking soda—it will bubble up a bit. Stir in the peanuts and immediately scrape the mixture out onto the prepared silicone mat or a large, rimmed nonstick baking sheet. Use a lightly oiled spoon or spatula to spread it out to make a thin, uniform layer.

Sprinkle with sea salt and let cool completely.

Store peanut brittle in an airtight container for up to 1 month.