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.