Preheat oven to 350 degrees. Line a square baking dish with aluminum foil and spray with cooking spray and set aside.
In a microwave safe bowl that will be big enough to hold all the ingredients, put the butter, then microwave it for 30 seconds to soften it (you want it melted a little and easy to work with, it may need an additional 30 seconds.)
Add remaining ingredients to the bowl that contains the now melted butter (flour, eggs, sugars, vanilla extract and pinch of salt) and stir until everything is combined.
Spoon blondie batter into the foil lined baking dish.
Spoon caramel sauce over the blondie batter in four rows or streaks, then run a butter knife through the lines to make the marble lines and bring the caramel into the batter a little (this isn’t necessary, you can just spoon caramel sauce over the top of the batter). Next, sprinkle the sea salt on top of the caramel.
Bake for 30 minutes or until a toothpick comes out clean. Remove from oven; let cool for 5 minutes, then transfer the blondies to a cooling rack (do this by lifting the edges of the aluminum foil). Allow to cool and then cut into squares, and watch them disappear!