For the Crust:
4 1/2 ounces (9 tablespoons) unsalted butter, room temperature, plus more for parchment
1/4 cup packed light brown sugar
1 1/2 cups all-purpose flour
1/4 teaspoon table salt
For the Chocolate Caramel:
10 1/2 ounces milk chocolate, chopped (2 cups)
1 1/2 cups granulated sugar
1/4 cup water
3 ounces (6 tablespoons) unsalted butter
1 cup heavy cream
1/2 teaspoon table salt
1 tablespoon sea salt; preferably fleur de sel
Preheat oven to 350 degrees.
Make the crust: Line a 9 inch square baking pan with parchment, leaving an overhang on all sides; butter parchment, excluding overhang.
Beat butter and brown sugar on medium speed until pale and fluffy, about 3 minutes.
Add flour and table salt, and beat until just combined.
Press dough evenly into pan, and bake until lightly browned, about 30 minutes.
Make the Chocolate Caramel: Place chocolate in a medium bowl.
Heat granulated sugar and water in a small saucepan over medium-high heat, washing sides of pan with a wet pastry brush to prevent sugar crystals from forming, until amber, 10 to 12 minutes.
Remove from heat.
Add butter, cream and table salt.
Bring to a boil, stirring until smooth.
Pour over chocolate.
Let stand for 2 minutes.
Stir to combine, and let stand until cool, about 10 minutes.
Pour mixture over crust.
Refrigerate at least 4 hours, or overnight.
Run a knife around edges; lift parchment to remove whole bar from pan.
Sprinkle with sea salt.
Trim edges, and cut into 16 bars.
Bars can be refrigerated for up to 3 days.