Preheat the oven to 350 degrees F. Line an 8x8-inch pan with parchment paper, spray with cooking spray, and set aside.
In the bowl of a stand mixer, mix the melted butter and brown sugar together until smooth. Add the egg and vanilla, and beat until combined.
Add the salt and flour, and mix until just combined. Scrape down the sides of the bowl as necessary.
Pour the batter into the prepared pan in an even layer. Dollop the dulce de leche on top of the batter at random, and use a knife to drag it around, swirling it throughout the blondies. Press the reserved pieces of chocolate and nuts into the top of the blondies.
Bake 25-30 minutes, or until the blondies are set in the middle.
Remove the blondies from the oven, sprinkle some sea salt on top, and allow them to cool completely in the pan before slicing.