In a small saucepan, melt the butter and water on high heat. Add the sugar and corn syrup. Wipe down the sides of the saucepan with a damp pastry brush to remove any loose sugar crystals. Cook the sugar mixture to 300 degrees F. Remove from the heat and set the pan over an ice bath for 5 to 10 seconds to stop the cooking. Remove from the ice bath. Add the vanilla and baking soda, stirring to incorporate. Quickly pour the mixture onto a silicone pad and spread thin. Sprinkle with chopped hazelnuts. Let set until completely cool, 5 to 7 minutes.
Once the toffee is set, turn over onto a sheet of parchment. Spread a thin layer of fluid, Tempered Chocolate onto the toffee and let set. Once set, the chocolate should be shiny, hard, and should snap with any given resistance. Turn back over and break into desired pieces.
Over a double boiler, melt 4 ounces chocolate to 115 degrees to 120 degrees F. Bring the bowl, with the chocolate, to a flat, clean work surface ( preferably marble or stainless steel ), wiping away any excess water or steam from the bowl. With rubber spatula, fully incorporate the remaining 1-ounce chocolate. Bring down the temperature of the chocolate to 80 to 84 degrees F by either stirring constantly in a figure 8 motion, or tabling (see Cook's Note). Once cooled, test the chocolate by dipping a small strip of parchment paper into the melted chocolate, and then lay it down on a clean work surface. If tempered, the chocolate on the parchment paper should be dry, hard, shiny, and without any streaks in 3 to 5 minutes. If any of these occur, keep stirring (agitating) the chocolate, or table it again, until a parchment paper test shows that the chocolate is tempered. When tempered, bring the chocolate back to the double boiler and reheat to NO HIGHER than 89 degrees F. (Bringing the temperature back up aids in a more fluid working consistency.) Transfer the chocolate to a chocolate warmer or set the bowl on top of a heating pad (at medium setting) to keep in temper.
To table, pour 2/3 of the chocolate onto a marble slab, quickly spread with a thin metal spatula, scrape into a pile with a putty knife, and then spread thin again. Repeat until the right consistency is reached. Add back to the warm chocolate.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.