To set up: Butter a baking sheet (about 10 by 12) and line the sheet with of parchment paper (allow excess paper at both ends). Grease paper and set sheet aside. Reserve any remaining melted butter for later. Line two more shallow baking sheets or trays with waxed paper for holding and chilling assembled candy and set them aside. Cut each stick of butter in half and place next to work surface, along with measured salt, baking soda, vanilla and coarsely chopped macadamia nuts. Place chopped chocolate in a heat proof bowl and set aside. Place a cup of hot water next to stove and put a pastry brush in water.

To prepare toffee: Place a 2 1/2quart heavy bottomed saucepan on the stove. Place water, sugar and corn syrup in the pan and stir gently to combine. Place pan over medium heat and as sugar dissolves, swirl pan gently. Let mixture come to a rolling boil and, once boiling and mixture is clear, don't stir or swirl pan anymore. If, while sugar is boiling, any sugar crystals cling to sides of pan, use moistened pastry brush to wash sides of the pan. Cook syrup until starts becoming a very light amber color. Lower heat and add butter, one half stick at a time, stirring very gently. (Only add additional butter after preceding piece has become melted and fully incorporated.) Once all the butter has been added, place candy thermometer into mixture, making sure that the bottom tip does not touch the bottom of the pan. Raise heat to medium and boil mixture until the temperature reads between 295 degrees F and 300 degrees F. Remove thermometer from pan and pan from stove and immediately stir in salt, baking soda, vanilla and macadamia nuts. (Quickly stick thermometer it warm water to aid in clean up.) Immediately, pour candy mixture onto prepared baking sheet. Brush the bottom of a long offset metal spatula (an icing spatula is perfect) or a knife with some melted butter. Spread the candy mixture out so that it is 1/4-inch thick. Use the knife to push the sides back into a rectangular shape as the mixture cools. Let the mixture set for a few minutes.

To melt chocolate: While toffee firms up a bit, melt chocolate in the top of a double boiler or in a skillet with 1inch of hot but not simmering water (stirring constantly until smooth). Be careful not to allow any water to enter bowl of chocolate.

To cut candy: While candy mixture is still warm and pliable, use a buttered pastry scraper to score the candy into 1 1/2 by 2inch rectangles. Let set further until the candy is firmer (5 minutes at room temperature). Score again through original cuts, making sure candy is in separate squares all the way through to the bottom. ( If stubborn, use your pastry scraper and mallet to knock through any sticky, tough spots. Hit the top (wooden part) of the scraper with a heavy mallet.)

To fully assemble candy and chill: When chocolate is firm, pour melted chocolate over top of scored candy and, using a spatula, spread it out evenly. It's okay if the chocolate runs down into grooves in candy. Scatter 2 cups of the ground toasted nuts on top of the chocolate and press them into the chocolate. Reserve the remaining 1 cup of nuts. Lift one end of the parchment paper so the first line of cut candy bends and separates from the next line. Carefully lift one piece off at a time. Sprinkle with some additional nuts to coat any exposed chocolate. Place candy on prepared baking sheets lined with waxed paper (in a single layer) and refrigerate (covered with plastic wrap) so chocolate will harden. Once set, place candy in an airtight tin, separated in layers with waxed paper. Although these candies may be stored at room temperature, to best preserve fresh flavor, store them in the refrigerator