Preheat oven to 350 degrees
In a mixing bowl, cream together 1/2 cup butter and 1/4 cup sugar
Mix in flour and milk and salt. It will look very crumbly.
Prepare an 8 x 8 baking pan by spraying it and lining it with parchment paper
Bake for 20 minutes, remove from the oven and let cool completely
In a heavy pan mix together 3/4 cup salted butter, 1/2 cup sugar, 3 Tbs light corn syrup, vanilla, and a can of sweetened condensed milk
Stir continuously, bring to a boil, and continue stirring until it turns a golden brown (about 6-8 minutes).
Pour over the top of the cooled shortbread crust
In a microwave safe bowl, melt the semi-sweet chocolate chips in 30 second intervals and spread over the caramel
In a microwave safe bowl, melt the white chocolate chips in 30 second intervals, and drizzle over using a spatula to make swirls, then sprinkle on some large crystal salt (optional)
Let cool completely, and then cut into bars for serving.
Can be stored for a week in an airtight container.