Preheat oven to 350℉. Line a 9×13″ pan with parchment or spray with non-stick cooking spray.
In a large bowl, combine the flour and brown sugar. Use a cheese grater to grate the butter and add it to the flour mixture. Combine the mixture with clean hands to form what resembles coarse crumbs, lightly breaking up butter pieces. Lightly beat the egg and pour it over the flour mixture and quickly combine it using a fork or clean hands.
Reserve ½ cup of the chocolate chips and add the remaining 1½ cups chocolate chips to the dough mixture, along with nuts if using. Measure out 1 ½ cups of this dough mixture and set aside. Lightly press the remaining dough into the prepared pan and bake for 10 minutes.
Remove pan from oven and pour the sweetened condensed milk over the hot crust, gently and lightly spreading it out without breaking up cookie dough below.
Reserve ¼ cup of the toffee bits and set aside and pour the remaining bits over the sweetened condensed milk.
Sprinkle the reserved 1½ cups dough mixture over the toffee bits and then sprinkle on the remaining ½ cup chocolate chips. Return to the oven and bake for 25-30 minutes or until golden brown. Mine usually take a little longer- like 30-35 minutes, but all ovens are different so keep an eye on them. When finished you should see them bubbling throughout, middle included, and the top should be starting to turn golden brown.
Remove from oven and top with remaining toffee bits. Cool completely and then cut into bars.
Notes
Store finished cookie bars in an airtight container at room temperature and enjoy within 3-4 days for best results. You can also store in the fridge if you prefer.
Don't be too hasty! Let the bars cool all the way before you cut into them for nice, clean edges.