The secret seems to be to cook the toffee mixture until it is dark enough. It starts out a pale yellow and gradually turns more brown. When it is brown like a grocery sack, it's ready. You'll know it's just long enough if you see a streak of very dark brown or smoke. Even with a little smoke, it still seems to turn out. Apparently other recipes for toffee say to cook it to 300 degrees. So you can always try it that way if you don't trust me. You'll know you've cooked the toffee just right because it will be crunchy but not stick to your teeth.
Also, use a heavy-bottomed pot. I've used cast-iron, an enameled dutch oven, and even a shallow skillet. A thinner bottom pot will burn the toffee before it's finished.
If you're a scaredy-cat and want to try some fail-proof candy, look here.
from my mother
2 sticks butter
1/2 tsp. salt (if using unsalted butter only)
1 c. sugar
3 Tbsp. water
milk chocolate or semi-sweet chocolate chips or bars broken into small pieces
finely chopped nuts (walnuts, pecans, or almonds)
Prepare two pie plates or one 9x13 dish. Set aside. In a heavy-bottomed large pot, add butter, salt (if using), sugar, and water. Cook on medium high heat and stir constantly. Cook until the mixture turns the color of a brown paper sack or reaches 300 degrees on a candy thermometer , but at the first sign of smoke or dark brown streaks, remove from heat. Immediately pour half of the mixture over each pie plate or all of the mixture in the 9x13 dish. Top with chocolate and all to sit for a few minutes until the chocolate is melted and spreadable. Spread the chocolate evenly over the toffee. Top with chopped nuts. Let cool completely. Use a butter knife to break apart into bite-size pieces of toffee.
Alternate method for double sided toffee: line a 9x13 dish with parchment paper and one other similar (or larger) sized dish (like a cookie sheet or another 9x13 pan). Follow all steps up to letting the toffee cool. As soon as you can after topping the toffee with chocolate and nuts, flip the pan over onto the second parchment lined dish. Add more chocolate, allow it to melt, spread it evenly, and top with more nuts. Let cool completely. Use a butter knife to break apart into bite-size pieces of toffee.