Caramel is a one-ingredient recipe for experts, two for more cautious cooks who add water to the sugar—but either way it can quickly turn into a chemistry experiment gone wrong. The problem is a rapid acceleration of browning, which can quickly move your sugar sauce into bitter, burnt territory.
Sugar behaves differently from other foods when it's cooked. While most ingredients absorb heat from the pan, sugar actually generates its own heat as it breaks down. This causes the temperature to rise fast—about one degree per second. When you remove the pan from the heat as the caramel reaches the perfect light-amber hue, it can still burn because residual heat from the pan keeps the action going.
The key is watchful, hands-off cooking, as slow and even as possible. Adding ¼ cup of water per cup of sugar dissolves the sugar uniformly and slows boiling, providing more control as you look for that honey-gold color. Use a light-colored stainless steel or enamel saucepan and a candy thermometer.
To make the caramel, cook the sugar and water, without stirring (or absolutely minimal stirring, if you must), over medium-low heat until golden and fragrant, about 335°. With experience, you'll learn to trust color more than temperature.
The hands-off approach works best because stirring can cause hot caramel to crystallize when it hits the cool sides of the pan, and that can set off a chain reaction that ruins the sauce.
Set the pan in an ice bath for two to three seconds to stop the cooking (any longer and the caramel will seize), then use immediately.