Prep the pan: Line a rimmed baking sheet or 9×13-inch pan with parchment paper or a silicone baking mat. Have this ready before you begin because the toffee needs to be poured immediately once it reaches temperature.
Melt ingredients: Add the butter, sugar, water, and salt to a small/medium, heavy-bottomed saucepan over medium-low to medium heat.
1/2 cup unsalted butter, 1 cup granulated sugar, 1 Tbsp. water, 1/4 tsp. salt
Bring to a gentle boil: Stir gently and continuously. Once the mixture begins gently bubbling, continue cooking over medium to medium-low heat, stirring steadily and scraping the bottom and sides of the pan so the mixture cooks evenly.
Cook the toffee: Continue cooking until the mixture reaches 300-305°F on a candy thermometer, about 10-12 minutes depending on your pan and stovetop Start Timer. As it cooks, the mixture will bubble and foam before thickening, smoothing, and becoming glossy. The finished toffee should be a rich golden-caramel color and smell deeply buttery and caramelized. Keep an eye on the heat: If the mixture browns quickly, smells burnt, or you notice dark spots forming on the bottom, reduce the heat. Slow and steady is better than rushing the toffee.
If the butter separates: Don't panic if you notice a layer of melted butter pooling around the edges. Remove the saucepan from the heat and carefully stir or whisk until the mixture comes back together. Return it to lower heat and continue cooking and stirring. Avoid suddenly increasing the heat, which can encourage the butter and sugar to separate.
Pour immediately: Pour the hot toffee onto the prepared pan. Use a heatproof spatula to quickly spread it into a thin, even layer. Don't worry about making it perfectly uniform since you'll be chopping it into bits.
Cool: Let the toffee sit undisturbed at room temperature until completely cool and hard, about 20-30 minutes. Start Timer
Make the toffee bits: Once completely cooled, break the toffee into pieces and chop with a sharp knife into small, irregular bits. For cookies, I like a mixture of tiny pieces and slightly larger chunks, so you get toffee throughout every bite.