Line a square 9x9 baking pan with foil, leaving the ends overlapping the edges so you can lift the toffee out when it has set for easier cutting. Spray with nonstick baking spray unless you have used nonstick foil.
Put the butter, sugar, corn syrup, water, and salt in a heavy saucepan. Bring to a boil over medium heat, stirring constantly at first while the mixture comes to a boil, then occasionally afterwards.
Gently boil the mixture, stirring occasionally , until it darkens in color and reaches 300F on a candy thermometer.
Remove from the heat and stir in the baking soda and 1 cup of the chopped walnuts. Pour the hot candy into the prepared pan, smoothing out with a spatula. Be very careful as this is very hot.
Scatter the chocolate chips evenly across the surface of the toffee and let sit for a couple of minutes to let the chocolate melt. Spread the chocolate out evenly with an offset spatula. Then top with the remaining 1/2 cup of chopped nuts, pressing them in gently so they will stick to the chocolate.
Let cool until set, either at room temperature, or in the refrigerator.
Remove the foil and cut into pieces. The toffee will probably crack into some uneven pieces as you slice, that's to be expected.
Notes
Store the toffee in an airtight container with parchment paper to separate layers. Will keep for up to 2 -3 weeks. Toffee can be frozen for longer storage, about 2-3 months for best quality.
When ready to enjoy, thaw the toffee in the refrigerator or at room temperature. Avoid microwaving, as it can cause uneven thawing and affect the texture.