How to Make It
Grease a 9-by-13-inch baking pan with 2 tablespoons of the butter and scatter the graham crackers evenly over the bottom.
In a large saucepan, combine the remaining stick of butter with the sugar, condensed milk and whole milk. Cook over moderately low heat, brushing down the side of the pan with a wet pastry brush, until the sugar is dissolved, about 12 minutes. Bring the mixture to a simmer, then cook over moderately low heat, stirring occasionally with a wooden spoon, until a pale caramel forms and the temperature reaches 240° on a candy thermometer, about 15 minutes.
Remove the saucepan from the heat. Add the vanilla and a generous pinch of salt to the fudge, and stir vigorously until it thickens but is still pourable, 8 minutes. Immediately scrape the fudge into the prepared pan and spread it evenly with a spatula. Let the fudge cool completely, 1 hour.
Invert the pan onto a surface and tap out the fudge. Break the fudge into pieces and serve.