Melt the butter in a heavy saucepan over medium heat. Add milk, sugar and syrup. Stir continuously with a wooden spoon until a candy thermometer reaches 240 degrees F. Take off heat immediately and stir in the vanilla.
Pour into your prepared pan. Cool in the refrigerator until firm. Pop caramel block onto a cutting board. Cut into 2 inch cubes and wrap each caramel in waxed paper.