Preheat oven to 350° and place oven rack in middle position. Grease and line an 8X8 baking dish with parchment paper and set aside.
Using a food processor combine cold butter, all purpose flour, powdered sugar, and salt. Pulse several times until butter pieces are very small, smaller than a pea. It will still look very powdery, that is ok. This can also be done with a pastry cutter or even your fingers but is much faster in a food processor.
Dump mixture into prepared baking dish and press into a tight even layer using the bottom of a cup or something similar. Make sure to pack down tightly. Bake in preheated oven for 12-15 minutes. The edges will be slightly golden brown. Set aside and make the filling, leaving the oven at 350°.
Pecan pie bar filling
In a large mixing bowl whisk together melted butter, light corn syrup, brown sugar, eggs, egg yolk, vanilla, and salt. Whisk until fully combined. Next, add in the pecans, chopped toffee or toffee bits and chocolate chips. Mix to incorporate evenly. Pour over previously baked shortbread crust. Place back in oven for 20-24 minutes. The center should be set and the filling bubbling all over. Top with flaky sea salt if desired. Allow to fully cool before slicing.
Notes
These bars are best eaten the day they are baked. They can be stored in an airtight container for two days and are best eaten within this time frame.