Line a baking tray with parchment and grease the parchment with butter.
Combine sugar, corn syrup and water in a heavy-bottomed saucepot and bring up to a boil over high heat. Without stirring, cook sugar until it reaches “hard crack”, about 300 degrees F. on a candy thermometer, brushing the sides of the pot occasionally with cool water. This will take about 7 to 9 minutes, and it will turn a deep amber colour.
Remove from heat and stir in salt and peanuts with a wooden spoon. Stir in butter, vanilla and baking soda – it will immediately get foamy. Working quickly, spread nut brittle onto prepared baking sheet. Allow to set completely before cracking into pieces.
Crack cooled brittle into pieces.
Melt 4 ounces of semisweet chocolate over a pot over gently simmering water while stirring, then drizzle on top of brittle. Sprinkle lightly with coarse fleur de sel and allow chocolate to set. (Fleur de sel is a French sea salt available in fine food stores.)