To make the shortbread, preheat the oven to 350 degrees.
In the bowl of a stand mixer, beat together the butter, sugar and flour until crumbly.
Press into a 9x9 baking dish and bake for 20 minutes.
To make the caramel, add the butter, brown sugar, corn syrup, and condensed milk to a medium sauce pan set over medium heat. Bring to a boil, stirring often, and continue boiling for 5 minutes. Remove from the heat and stir vigorously for 3 minutes.
Pour caramel over the (warm or cool) shortbread.
For the topping, sprinkle the fleur de sel over the caramel.
Sprinkle the chocolate chips on top and return to the oven for 1 minute.
Spread the chocolate chips with an offset spatula or swirl with a knife for a marble effect.
Cool completely and cut into small squares.