Preheat the oven to 350°F.
Using a stand mixer or hand mixer, cream the butter, sugar, egg yolk and vanilla, then stir in the flour and salt until well mixed.
Pack the crust into a 9x13 greased or parchment lined baking sheet. (Notes from Shelly: I make it into a rectangle, leaving 1 inch because it will come to the edge as it bakes – I use my fingers. You want it to be about ¼-½ inch thick. If it is to thick, it won't cook evenly.)
Bake 20 to 25 minutes until nicely browned.
Take the toffee cookie bars out of the oven (the crust will be soft!), immediately place the broken chocolate on top, and let it stand until softened. Spread the chocolate evenly over the crust.
Sprinkle the chocolate with chopped pecans (or any other topping you want to add). Then, cut into small squares while warm – a pizza cutter is perfect for this.
Let the toffee squares finish cooling to harden, then serve and enjoy!