Classic Hard Butter Toffee

Prep the Landing Zone: Line a 9×13 inch baking sheet with parchment paper or a silicone baking mat. Do not skip this, or the toffee will become a permanent part of your pan.
The Melt: In a heavy-bottomed saucepan, combine the butter, sugar, and water. Cook over medium heat, stirring constantly with a wooden spoon or silicone spatula until the butter is melted.
The Boil: Once it starts boiling, stop stirring so much—just occasional swirls. Attach your candy thermometer. You are looking for the mixture to turn a deep golden amber (the color of a paper grocery bag).
The Critical Temp: Continue cooking until the thermometer reaches 300°F (149°C). This is the “Hard Crack” stage. If you go too far past this, it will taste burnt; if you stop early, it will stick to your teeth.
The Finish: Immediately remove from heat. Stir in the vanilla extract (it will bubble violently, so be careful).
The Pour: Pour the molten toffee onto your prepared pan. Use a spatula to spread it to your desired thickness.
Set & Snap: Let it cool completely at room temperature (about 1 hour). Once hard, use a knife to score it into the squares seen in your photo, or simply break it into rustic shards by hand.

The “Clean” Rule: Make sure your saucepan is spotlessly clean. A single grain of old sugar can cause the whole batch to “crystallize” and turn into a grainy mess.
Don’t Rush: If you crank the heat to high, the butter might separate from the sugar (leaving a pool of oil). Keep it at a steady medium.
Humidity: Toffee hates humidity. If it’s a very rainy day, your candy might stay slightly sticky.