Generously grease bottom and sides of a jelly-roll or half-sheet pan; set aside.
In a large, heavy pan, melt butter over medium-low heat. Add sugar, corn syrup, and water. Stir until sugar dissolves. Cook very slowly to 310°F on a candy thermometer, stirring occasionally.
When toffee reaches 310°F on the candy thermometer, add vanilla, stir well, and pour into the prepared pan. Allow toffee to cool to the touch, then gently dab the surface of the candy with a clean paper towel to remove excess butter.
Pour melted chocolate over candy and spread evenly over entire surface. Immediately sprinkle nuts over top and press down slightly to help nuts adhere.
Once chocolate has set, break toffee into large pieces. Store at room temperature in a loosely covered container.
Notes
Keep in mind that humidity affects candy-making. If you try making this candy on a rainy day, the toffee will not set.