Preheat the oven to 350℉. Line a 9x9-inch baking pan with parchment paper, allowing the paper to hang over the side of the pan.

In a medium-sized bowl using an electric handheld mixer on high speed, cream the softened butter and powdered sugar together for 1 minute. The mixture should be smooth and creamy. Add the flour and mix at low speed until incorporated, the mixture should be crumbly. Stir in the chopped pecans. Press the mixture firmly into the prepared baking pan to make a crust. Bake in the preheated oven for 18-20 minutes or until slightly golden around the edges. Let the finished crust cool slightly while you prepare the caramel.

Melt the salted butter in a heavy saucepan over medium heat. Continue cooking over medium heat, stirring frequently until the butter is chestnut brown in color and has a nutty aroma. It is completely normal for the butter to sputter, bubble, and foam during the process. Turn off the heat and let cool slightly in the saucepan for about 10 minutes.

Add the sweetened condensed milk, brown sugar, and corn syrup to the slightly cooled brown butter in the saucepan and whisk to combine. Turn the stove on to medium heat and cook the mixture for about 5 minutes in total, while whisking constantly. The mixture should boil and turn a light caramel color. Take the finished caramel off the heat and whisk in the vanilla before immediately pouring over the finished, slightly cooled crust, spreading well into the edges of the pan and smoothing the top. Let cool to room temperature before refrigerating for at least 1 hour to firm up.

Place the chopped dark chocolate in a heat-safe bowl. Heat the whipping cream to scalding and pour over the chocolate. Let sit for 2 minutes before whisking briskly until smooth.

Pour the ganache over the chilled bars and smooth out the top as best as you can. Top with flakey sea salt, if desired. Cover and return to the refrigerator for at least 1 hour to allow the ganache to firm up.

To serve, remove the bars from the pan using the overhanging edges of parchment paper as handles. Slice in 16 pieces using a sharp knife. Serve chilled or allow to warm up on the counter for 15-20 minutes if you prefer a softer caramel. Enjoy!