Grease a rimmed baking sheet with butter. Put it on the counter close to the stove.

Put the sugar and 1/3 cup water in a medium saucepan over low heat. Cook, without stirring, until the sugar dissolves and becomes a nutty caramel color but it's not yet dark brown, 5 to 10 minutes; swirl the pan if the sugar is cooking unevenly.

If there's sugar clinging to the sides of the pan, use a pastry brush dipped in water to wash it back down. Watch carefully, as sugar has a tendency to burn the second you turn your back on it.

Stir in the peanuts and salt if you're using it until combined. Remove from the heat and immediately pour the candy onto the prepared baking sheet and tilt the pan if it's not spreading fast enough to coat the bottom.

Let cool at room temperature.

To make even squares, score the brittle with a sharp knife once it's solidified slightly but before it has hardened, then cut when completely cooled. Otherwise, break it into pieces when cool. Store in a covered container at room temperature indefinitely.