Line a standard size baking sheet with parchment paper or a silicone baking mat, set aside.
In a large, heavy bottomed saucepan, add the sugar, corn syrup, water, and salt. Turn heat on to medium and bring the mixture to a simmer, whisking frequently.
Continue whisking until the temperature reads 280 degrees F on a candy thermometer (this can take upwards of 10 minutes). Add the butter and peanuts. The candy will bubble and then stiffen up, this is normal.
Continue whisking until the candy reaches 300 degrees F on a candy thermometer (this can take anywhere from 8-12 minutes). Remove from the heat and whisk in the vanilla and baking soda. NOTE: the candy will not look dark like it will once it sets.
Quickly pour the mixture onto the prepared baking sheet. Using a offset spatula (or by tilting the pan, just use gloves because it will be extremely hot), spread brittle into a smooth, even layer from corner to corner.
Allow the brittle to set until hard, this takes about an hour or so depending on room temperature and humidity. Once set, us your hands or the back of a metal utensil to break the brittle into chunks.
Store in an airtight container at room temperature for up to 5 days.