Preheat oven to 350 degrees. Butter two 8-inch-square baking pans; coat with flour, tapping off excess. Place flour, sugar and kosher salt in food processor; pulse once. Add 1 cup butter; pulse until mixture resembles peas. Dividing evenly, press mixture into pans. Bake 18-20 minutes or until golden around edges. Cool completely.
In heavy-bottomed pot over medium-low heat, combine condensed milk and remaining 2 T. butter. Slowly bring mixture to boil, stirring continuously. Continue stirring over heat until mixture becomes thick and amber in color, 15-20 minutes. Pour caramel over shortbread; spread evenly. Cool to room temperature.
In heatproof bowl set over pot of hot, not simmering, water, melt chocolate. Once chocolate has melted, pour it over caramel layer. Sprinkle a little fleur de sel over chocolate to garnish. Cool at room temperature for about 10 minutes. Refrigerate to set. Cut into 2-inch squares.