Lightly grease a 10 x 15-inch baking sheet. Set aside.
In a large heavy saucepan, combine the brown sugar, butter, and water and bring slowly to the boil, stirring occasionally to prevent burning. Insert a candy thermometer and cook until the mixture reaches 285 degrees F.~ (soft-crack stage). Remove from the heat and quickly stir in the vanilla and baking soda. Immediately pour onto the greased baking sheet. Sprinkle the chocolate chips over the hot toffee. Let sit for 5 minutes, then spread evenly over the surface of the toffee with a spatula. Sprinkle the nuts evenly over the top. Cool completely. Break the hardened toffee into pieces and store in an airtight container.
