Preheat the oven to 350°F. Line a 9x13 baking dish with parchment paper and spray with cooking spray, set aside.
Place the 1 1/3 cups unsalted butter and 1/2 cup sugar into the bowl of a stand mixer with the paddle attachment or a large mixing bowl using an electric hand mixer, and cream together for 3 minutes until light and fluffy.
Add the 2 1/2 cups flour and 1/8 teaspoon fine sea salt, stir it in until combined with no dry patches.
Press the dough into the prepared baking dish. Bake for 23-28 minutes until lightly golden brown along the edges, the center of the crust appears dry, and a toothpick inserted into the center comes out clean. Let cool completely.
Filling:
Combine the 1 cup unsalted butter, 1 cup light brown sugar, 1 cup sweetened condensed milk, and 1/4 cup light corn syrup into a medium-sized sauce pot. Place over medium-low heat, stirring constantly.
Bring to a simmer and turn the heat to low. Gently simmer for 5 minutes, stirring constantly so nothing burns on the bottom. You really have to be careful and make sure it doesn’t boil because a bubble may pop and burn you. If it looks like it is starting to simmer more rapidly, take off the heat for a few seconds while continuing to stir, then place it back on the heat.
After 5 minutes, take it off the heat and continue to stir for about 4 minutes to cool it down slightly. Pour over the crust evenly. Place in the freezer for 2 hours.
Chocolate Layer:
Add the 12 ounces semi-sweet chocolate bars and 2 tablespoons unsalted butter into a microwave-safe bowl. Microwave in 15-second intervals stirring in between until smooth. Spread the chocolate on top of the filling. Place in the fridge for 2 hours to set.
Use the parchment paper to help lift the bars out of the baking dish. Cut into squares and sprinkle with flaky sea salt for garnish, if using, serve immediately.
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