Line a large baking sheet with parchment paper or a silicone baking mat and set aside.

In a large heavy saucepan, combine the sugar, water, and light corn syrup. Cook over medium heat, stirring occasionally, until the sugar dissolves completely.

Attach a candy thermometer to the side of the pan and continue cooking until the mixture reaches 245°F.

Stir in the peanuts and salt if using. Continue cooking, stirring frequently, until the temperature reaches 300°F and the mixture turns a deep golden color.

Remove the saucepan from heat. Quickly stir in the butter, vanilla extract, and baking soda. The mixture will foam and lighten in color.

Immediately pour the hot peanut brittle onto the prepared baking sheet and spread into a thin, even layer.

Allow the brittle to cool completely at room temperature.

Once fully cooled, break into pieces and serve.