Have your roasted peanuts, salt, a wooden spoon, a rolling pin, and 2 sheets of parchment paper ready on the counter.
Place sugar and water in a medium-small heavy-bottomed sauce pan (heavy-bottomed is important, otherwise the sugar is more likely to burn. I used one that's 2.5QT). Warm on medium high heat until the sugar is dissolved into the liquid, stirring occasionally. Once mixture begins to simmer, lower heat (I had mine set somewhere between 3 - 4 out of 9 settings). Stick a candy thermometer in the pan and allow the mixture to cook until it reaches 275F (close to hard crack stage). Keep a close eye to make sure the mixture doesn't burn towards the end of the process.
Once sugar mixture is ready, take it off the heat, quickly stir in salt and peanuts. Scoop the mixture out onto a sheet of parchment paper. Cover with a second piece of parchment and roll out into a flat shape with a rolling pin (it helps to hold the edges of one corner in place by pressing it to the counter with your waist / hip as you roll away from yourself). Allow the brittle to cool an hour or two -- until dry. Remove the parchment paper and snap the brittle into pieces. Enjoy! Store in an air-tight container at room temperature or refrigerate.