Preheat oven too 350 degrees and prepare 9X9 pan with parchment paper to aid removal when finished
Cream butter and sugar and add egg, vanilla and salt and beat until light and fluffy
Slowly add flour and mix well
Press batter into pan and bake for about 20 minutes
While base is baking, make the caramel by combining the butter, brown sugar, vanilla, sweetened condensed milk and white syrup in a sauce pan.
Over medium heat, whisk until thoroughly while bringing to a boil. Coontinue whisking until candy thermometer reaches 225 degrees. (about 5 minutes)
Pour caramel over the shortbread base, leveling with spatula. Put in the fridge for at least 10 minutes or longer to set the caramel.