Line an 8-inch square baking dish with parchment paper.

In a heavy-bottomed saucepan, combine sugar, condensed milk, butter, and milk.

Heat over medium heat, stirring constantly until sugar dissolves.

Bring the mixture to a boil, stirring continuously.

Reduce heat to medium-low and continue to cook, stirring constantly, until the mixture reaches 115°C (240°F) on a candy thermometer.

Remove from heat and stir in vanilla extract.

Beat the mixture vigorously with a wooden spoon until it thickens and starts to lose its glossy appearance.

Pour the mixture into the prepared baking dish.

Let cool completely before cutting into squares.