1 preheat oven to 325° F.

Spray a 10×10 baking dish with non-stick cooking spray.  Line with parchment paper.

Place butter in a medium saucepan over medium heat.  Melt butter and continue stirring until butter becomes amber in color.  Transfer to a large heat-proof bowl and immediately add chocolate chips, cocoa powder, Mexican chocolate and vanilla.  Stir until chocolate chips melt and chocolate is smooth and glossy. Let cool for 5 minutes.

Place eggs and sugar in a bowl of a stand mixer.  Whisk on medium-high speed until a creamy and fluffy consistency; about 2 minutes.

Reduce whisk speed to low and add the melted chocolate mixture and salt. Remove bowl from mixer and fold in flour; Do not over mix.

Pour batter on to prepared baking dish. Using a spatula, smooth and level the top of the batter.

Bake for 45-50 minutes on center rack or when tested with a toothpick in the center, the toothpick comes out with only a few crumbs.

Remove baking dish from oven, sprinkle with fleur de sel (optional) and let cool for 20 minutes. Transfer to a wire rack and let cool completely before slicing.