Position rack in center of oven and preheat oven to 375 degrees F.
Spread nuts on a baking tray and bake until toasted, 10 to 15 minutes, turning nuts after 7 to 8 minutes to ensure even toasting. Cool and chop coarsely. You should have about 2 cups. Set aside. Clean baking tray and coat with vegetable oil or vegetable oil spray. Spray both sides of the offset spatula with vegetable oil spray. Set aside.
Make the toffee: Place sugar in a small deep saucepan. Add the corn syrup and water or rum and, over medium heat, bring to a boil. Let boil until large bubbles form on the surface, 3 to 4 minutes. Cover with foil and boil 5 minutes longer.
Add the pieces of butter and continue cooking, uncovered, over medium heat until temperature reaches 300 degrees F on a candy thermometer, about 30 minutes. (The candy thermometer has a clip on the side that can be slid up or down. If desired, you can fit the thermometer on the side of the pan, making sure that it rests in the mixture, not on the bottom of the pan, so that the reading is accurate.) Remove from heat and immediately add the salt, baking soda, and 1 cup of chopped nuts. Stir with a wooden spoon until well combined.
Pour the toffee mixture on the oiled baking tray and, using the offset spatula, spread the mixture out, making a block about 10 by 13 inches. The toffee will thicken very quickly, so work as fast as you can. When cool, blot some of the oil or vegetable spray with a clean towel.
While the toffee mixture is cooling, melt the chocolate in a medium heatproof bowl placed over simmering water. When almost melted, turn off heat and let the chocolate continue to melt, stirring occasionally. Keep warm.
When you can lift up the block of toffee, transfer it to a flat work surface or to a clean baking tray. Pour the melted chocolate over the toffee, spreading to cover the entire surface and, before the chocolate cools, sprinkle with the remaining chopped nuts. If the chocolate takes a long time to harden, refrigerate for about 10 minutes to set the chocolate.
Return the block of toffee to the work surface and break up into pieces approximately 1 to 1 1/2 inches wide. Then cut into smaller pieces, as desired. Use as needed.