Preheat oven to 400 degrees Fahrenheit. Spread parchment paper over a rimmed cookie sheet, covering the sides/edges too. (A half sheet/jelly roll pan works best.)

Place crackers in a single layer, close together, on top of the parchment paper.

In a saucepan, combine the butter and brown sugar over medium-high heat, stirring occasionally. 

Bring to a full, rolling boil (that won't go away when you stir). Turn heat down slightly to medium/medium-low (so there is still a boil, but not so high that it caramelizes too quickly). Set a timer for 3 minutes and stop stirring. After 3 minutes, immediately pour the sugar mixture over the saltine crackers. (Try to pour some on each cracker, but it will spread more in the oven so don't worry.)

Bake at 400 degrees Fahrenheit for 7 minutes. (After about 3 minutes, it will start to bubble and cover the crackers better.)

Remove from the oven and sprinkle chocolate chips over the top. Return to the oven for 2 minutes. Remove from the oven and let the chocolate chips soften for 1-2 minutes. Use an offset spatula to spread the melted chocolate evenly over the toffee. 

Immediately top with chopped nuts while the chocolate is warm. (*Toasted nuts taste better. You can place them in a skillet over medium heat and stir them until they get nutty and fragrant. About 3-5 minutes.) Cool completely and break into pieces.