Preheat oven to 350. Grease an 8x8 (2 qt) glass baking dish. Whisk the sugar and flour together. Cut in the 3/4 C (12 T) room temperature butter, with a fork, knife or pastry blender. Spray hands with cooking spray to avoid mixture sticking to hands (optional but helpful). Press shortbread mixture evenly on bottom of glass baking dish. Bake for 25-30 minutes or until edges are golden brown. Remove from oven and let cool completely before adding caramel layer. You could prepare this layer the day before to save time the following day.

Melt on medium low heat, butter and sugar, stirring well. Once completely melted, add the corn syrup, sweetened condensed milk and vanilla. Raise temperature to medium and bring mixture to a boil while stirring consistently. Once it begins to boil start a timer for 5 minutes. Continue to stir while boil also increases. See tips above. Turn heat off, remove pan from burner and continue to stir for a few minutes. The mixture should hold together, but be "pourable". Pour onto cooled cookie layer and spread evenly. The caramel can take 20-30 minutes to set. You can speed up this process by placing in the freezer for about 10-15 minutes. Once set, remove and set on counter top to prepare for chocolate layer.

Using a double boiler, or microwave, combine the chocolate chips and vegetable oil (or shortening), heat and stir til smooth. Pour over caramel layer, and let set up. You can speed this process along by placing in the freezer for 20-30 minutes. I get a serrated knife and cut into 25 bars (5 rows, 5 columns), immediately after removing from the freezer, and that helps me get a nice clean cut.