Yield: 32 pieces.
1½ cups granulated sugar
1 cup light brown sugar, firmly packed
⅓ cup heavy cream
⅓ cup whole milk
Pinch of salt
2 tablespoons butter, plus more for the dish and saucepan
1½ teaspoons vanilla extract
1. Butter an 8 x 8-inch baking dish. Line the dish with parchment paper, leaving a 2-inch overhang on all sides. Butter the paper on the bottom of the dish.
2. Butter the inside of a heavy 2-quart saucepan. Add the granulated sugar, brown sugar, cream, milk, and salt. Cook over medium heat until the sugars dissolve and the mixture comes to a boil, stirring constantly. Reduce the heat to medium-low. Clip a candy thermometer to the side of the pan so that the tip is immersed in the liquid. Continue cooking and stirring until the liquid reaches 235º (soft-ball stage), about 20 minutes. Gently remove the pan from the heat without jiggling the mixture. Leave the thermometer in place. Drop in the butter and vanilla.
Do not stir!
3. Let sit undisturbed for about 2 hours, until the temperature drops to 110º. Remove the thermometer.
4. Beat the fudge for 7 to 9 minutes with a handheld electric mixer set to low speed (or, if you have the stamina and elbow grease, by hand with a wooden spoon) until the fudge loses its gloss and turns smooth and creamy. Don’t give up hope: Nothing happens for a long while, then the fudge suddenly changes consistency. Do not overbeat the fudge or it will turn dry and crumbly.
5. Scrape the fudge into the prepared dish, and smooth the top. If the fudge is sticking to the spoon, lightly butter your fingers and press it into place. Let sit undisturbed and uncovered at room temperature overnight. Cut the fudge into squares. Store in an airtight container at room temperature for up to one week. Some cooks store the cut fudge in the refrigerator for longer storage, but others believe that chilling makes the fudge less creamy.