Brown the butter in a light colored medium sized pot, place butter inside and cook on low medium heat. No need to stir consistently, just allow the butter to melt and begin cooking.
About half way through the butter will begin to foam up a bit, but you only need to take your rubber spatula and give it a stir to bring it back down a bit. Continue to cook on the same heat, and about 6 to 7 minutes in you will begin to see light foaming on the top.
You will notice the along the edges browning, and if you stir the butter there will be small browned pieces at the bottom. Begin stirring at this point until the color is a light to medium color and you smell the nutty flavor of the butter.
Remove from heat and pour into heat safe bowl. Add dark chocolate morsels to hot butter and make sure they are completely covered. Allow to sit for 2 minutes.
Preheat oven to 350 F. While chocolate sits in hot butter, in a separate bowl combine flour, espresso powder, cinnamon, salt and cayenne pepper.
Sift in cocoa powder to remove any clumps.
Whisk dry ingredients until well dispersed. Set aside.
Mix butter and chocolate mixture until smooth and shiny. Set aside.
In a separate bowl combine eggs, sugar and vanilla. Then grate the mexican chocolate into bowl.
Whisk egg and sugar mixture vigorously until well combined - about 2 minutes.
Slowly add butter chocolate mixture into egg sugar mixture, making sure to whisk as you pour.
Add dry ingredients to wet ingredients.
Use a rubber spatula to gently fold dry ingredients into wet ingredients. Continue to fold until all flour has been incorporated.
Line a 8x8 square baking pan with parchment and baking spray.
Pour batter into baking pan and use spatula to spread evenly.
Bake for 22-23. Remove from oven and allow to cool before removing from baking pan. Top with flaky sea salt if desired. Allow to cool completely before cutting into brownies.