Prep Your Pan: Butter a 9-inch square baking dish and set it aside. This will prevent the toffee from sticking while it cools.
Make the Toffee: In a large saucepan, melt 1 cup of butter over medium heat. Once melted, stir in the sugar, Lyle’s Golden Syrup (or corn syrup), and water. Stir until smooth.
Heat to Perfection: Attach a candy thermometer to the side of the saucepan and cook the mixture to 290°F, the "hard crack" stage. Stir occasionally, but keep a close eye on the temperature.
Add the Walnuts: Once you hit 290°F, stir in 1 cup of chopped walnuts and cook for an additional 3 minutes, stirring constantly to prevent burning.
Pour and Cool: Pour the toffee into the prepared pan, spreading it out evenly. Let it cool at room temperature.
Melt the Chocolate: While the toffee cools, melt the 6 oz of semi-sweet chocolate chips in a heatproof bowl set over a pan of simmering water, or in the microwave, stirring frequently until smooth.
Top It Off: When the toffee is cool and firm, remove it from the pan and place it on a sheet of waxed paper. Spread half of the melted chocolate over the toffee slab, then sprinkle with half of the remaining walnuts. Let it set until the chocolate hardens.
Flip and Repeat: Turn the toffee slab over and spread the remaining melted chocolate on the other side. Sprinkle the rest of the walnuts on top and allow the chocolate to set again.
Break Into Pieces: Once the toffee has completely cooled and the chocolate is set, break it into bite-sized pieces. Enjoy your homemade Christmas Walnut Toffee!