Line a 9 x 13 pan with foil and spray with cooking spray.
Shortbread
In a food processor pulse together 1 1/3 C butter, white sugar and flour. Pulse until crumbly and combined.
Press mixture into prepared 9 x 13 pan and bake for 20 minutes or until just beginning to brown. Poke with a fork after removing from oven. Cool and then freeze for 30 minutes
Caramel
In a heavy 2 qt pan combine 1 C butter, brown sugar, corn syrup and sweetened condensed milk. Bring to boil and boil for 5 minutes. Then remove from heat and beat for 3 minutes with a wooden spoon. It will thicken up.
Remove shortbread from freezer and pour caramel mixture over the top. Refrigerate until firm and cooled.
Chocolate
Melt chocolate in a microwaveable bowl for one minutes, check and then continue for 30 seconds at a time until melted. Add in butter and stir until butter melts and chocolate is a good spreading consistency.