1. Cream together butter and brown sugar in a large mixing bowl.
2. Add egg yolk and vanilla and beat until combined.
3. Add salt and flour. Mix on low just until combined and dough comes together well.
4. Dump into ungreased baking pan. I use a 15 x 9 inch pan with sides. (Use a bigger pan and your toffee bars will be thinner and crunchy like my Mom prefers.)
5. Pop the tray of dough into the refrigerator and turn on the oven to 350 degrees. The dough can chill a bit while the oven is heating and you can clean up the kitchen.
6. Place the tray on center rack of pre-heated oven and bake for 18-20 minutes. Watch closely! If they brown too fast they will be crispy.
7. As soon as the bars come out of the oven dump entire bag of chocolate chips on top. Allow them to soften for five minutes then spread with offset spatula.
8. CUT BARS IMMEDIATELY after frosting. Once cut into squares allow to cool completely before storing. If your kitchen is warm they can go into the fridge to firm up the frosting before placing between layers of waxed paper in sealed containers.
These bars keep beautifully for several days at room temp and can also be frozen for 3-4 weeks prior to Christmas.