In a microwave safe bowl, add the butter. Heat on low until butter is just melted. Remove from microwave and add cocoa powder. Whisk until smooth then add your cocoa+butter mixture to the bowl of a stand mixture.
To the mixing bowl add both sugars. Mix ingredients until combined. Add the eggs one at a time on low, then mix for 5 minutes on medium speed until light and fluffy. Scrape the bowl incrementally to ensure even mixing. Add the vanilla and mix to combine.
In a bowl or ziptop bag add the salt, baking soda and flour and whisk to combine. With the mixture on the lowest setting, slowly add the flour mix until it's mostly incorporated. Be careful not to overmix. Turn off mixer and add chocolate chips/chunks by hand with a spatula.
Line a baking pan with parchment paper. Use a cookie scoop (or two-tablespoons of dough) and divide the dough into balls. Line on the pan about 3" apart and top with additional chocolate. Save the speciality chocolate pieces for later...
Chill the cookie dough balls for at least 30 minutes in the fridge before baking. (If the pan doesn't fit in the fridge, just put them on a plate.) Preheat your oven to 350 degrees.
While the cookies are cooling, chop a bar of good quality chocolate into 1/2-1" chunks. Set aside.
Bake the cookies for 5 minutes, pull out oven rack and rotate the cookie pan, then carefully top each cookie with chunks of speciality chocolate. Bake for another 3-4 minutes.
Allow the cookies to rest on the pan for a few minutes, then add a pinch of fleur de sel evenly to the tops of each cookie. Let them continue to rest for a 10 full minutes before transferring to a cooling rack. Enjoy immediately or store the cookies in an airtight container