While the panna cotta is in the fridge, place a long sheet of aluminum foil alongside the stovetop. Iin a heavy skillet over medium heat, mix the sugar with 3 to 4 teaspoons water. You want it to achieve the consistency of wet sand. Cook, occasionally tilting the skillet to swirl the mixture to ensure even browning, until the sugar is a deep caramel color, 5 to 10 minutes, depending on the size of your skillet. Do not stir the mixture. Working very quickly, remove the skillet from the heat, stir in the peanuts, and immediately pour it onto the foil and let it spill out into a flat amorphous blob. Let cool completely.