Line a large baking sheet with parchment paper. (Assumption: lightly grease the parchment or a silicone spatula for easier spreading.)
In a heavy-bottom saucepan, combine sugar, water, corn syrup, butter, and kosher salt. Heat over medium to medium-high until boiling.
Once boiling, cook without stirring for 8 to 12 minutes (or longer if at a lower temperature) until deep golden caramel (older penny color), about 390°F (as noted).
Remove from heat and quickly stir in the peanuts until coated.
Stir in vanilla extract, then immediately stir in baking soda (it will foam).
Pour onto the prepared baking sheet and spread into a thin, even layer quickly.
Cool completely until hard, then break into pieces.