Brown the butter by melting the butter in a small-medium sauce pan. Heat over medium heat, swirling and scraping the bottom of the pan with a rubber spatula to keep the butter solids from scorching. Cook the butter until it is golden and smells nutty. Remove from the heat, and allow to cool until room temperature. You may optionally sieve or strain the butter solids out of the butter if they have browned too much.
Sift buckwheat flour, sea salt, baking soda and cinnamon in a medium bowl (large enough to hold the entire batter once mixed). Set aside.
Once the browned butter is cool, add in the sugars, mixing in the pan the butter was browned in. Add in the egg and beat very well until combined.
Add the butter-egg-sugar mixture to the dry ingredients, and mix just briefly. Add in the chocolate and toasted nuts (if using), and mix to combine everything until no patches of flour are seen.
Cover the bowl, and refrigerate for at least 1 hour until firm.
To bake, pre-heat oven to 350F and line 2 baking sheets with parchment or silpat. Take 1 heaping tablespoons of batter and roll them into balls (or scoop using a cookie scoop). Place 2 to 3 inches apart, as the cookies do spread a bit.
Top each cookie with a sprinkle of sea salt. Bake for 10-13 minutes, until the edges are set and middles are still a bit soft but slightly collapsed. The cookie will be nicely browned on the bottoms, and slight browning on their tops.
Remove from oven and allow to cool until just warm - the cookies are fragile when hot.