Preheat the oven to 350°F. Line a 9x13 baking dish with parchment paper and spray with cooking spray, set aside.
Place the butter and granulated sugar into the body of a stand mixer with the paddle attachment, and cream together for 3 minutes until light and fluffy.
Add the flour and sea salt, and 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.
To make the filling place the butter, brown sugar, condensed milk, and 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.
Add the chopped chocolate and 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 flakey sea salt if using, serve immediately.