Line a baking tray with parchment and grease the parchment with butter.
Combine the sugar, corn syrup, and water in a heavy-bottomed saucepot and bring up to a boil over high heat.
Cook the sugar until it 275°F on a candy thermometer. (This will take about 7 to 9 minutes, and it will turn a deep amber colour.)**
Add the peanuts and pretzels and bring the mixture up to 300°F.
Remove from the heat and stir in the butter, vanilla, and baking soda – it will immediately get foamy, so stir madly!
Working quickly, spread the brittle onto prepared baking sheet.
Sprinkle with the flakey sea salt.
Allow to set completely before cracking into pieces