Grease a 9-by-13-inch pan and line it with parchment. Use a smaller pan for thicker squares or a larger one for thinner pieces.

Put the sugar, whole milk and butter in a large microwave-safe bowl. Microwave in 30-second bursts, stirring between bursts, until the butter has melted and the sugar starts to dissolve.

Pour the mixture into a deep stockpot or saucepan. Bring it to a boil over medium-high heat, stirring occasionally. Once the sugar dissolves, add the sweetened condensed milk and turn the heat down to medium. The syrup will foam up quickly.

When the mixture returns to a boil, lower the heat to keep it at a gentle simmer. Stir as it cooks for 15 to 20 minutes. Once the syrup thickens and turns caramel-colored, remove the pot from the heat when a candy thermometer reads 240–248°F. To check without a thermometer, drop a little syrup into cool water. It’s ready if the cooled drop holds together in a soft ball.

For an optional flavor variation, stir in 1 teaspoon of vanilla extract or 1 tablespoon of whiskey off the heat.

Beat with a hand mixer for 8 to 10 minutes, watching for the mixture to thicken and the beaters to leave trails. Stop as soon as you see the trails. A wooden spoon also works and may take longer.

Pour into the lined pan and tap it gently on the counter to settle the candy into the corners. Leave the surface alone. After 10 minutes, score it into squares. Let it set for 2 to 4 hours or overnight before cutting through the score lines.