Special tools:
-heavy duty stand mixer (again, hand mixer will do in a pinch)

Ingredients
2 C. Jeanne's Gluten-Free Flour mixture (or regular flour for non-gf)
3/4 tsp. baking soda
1 tsp. baking powder
3/4 tsp. coarse salt (like kosher salt)
1 1/4 sticks (10 TBL) unsalted butter at room temperature
1/2 C. dark brown sugar
1/2 C. granulated sugar
1 large egg
1 tsp. pure vanilla extract
12 oz. chocolate disks or feves (Valhrona+ E. Guittard make these), at least 60% cacao content
Sea salt for sprinkling

-cream butter and two sugars together using a mixer with a paddle attachment until fluffy--at least 5 mins.
-add egg, mix to combine
-add vanilla, mix to combine
-in a separate bowl, combine flour, baking soda, baking powder, and salt
-add dry ingredients, mix just until combined
-add chocolate disks by hand, incorporate (try not to break the pieces)

-cover with plastic wrap, pressing the wrap against the dough, and refrigerate for 24 to 36 hours. Can be refrigerated up to 72 hours (although I went away for several days and came back, made the cookies, and they tasted fine).

-when ready to bake, preheat oven to 350 degrees
-line baking sheet with parchment paper or Silpat mat
-scoop up dough into golf ball-sized amounts (this is hard because the dough will be hard and cold--I had to use a butter knife to carve out chunks of dough to work with. I then shaped these into balls with my hand).
-place on baking sheet
-press down each cookie a bit flatter with the bottom of a glass (not too flat, just so they're not ball-shaped)
-sprinkle each cookie with sea salt (at first I used flake-type sea salt--I found these to be a bit too salty, though other people liked them. Then I used coarse sea salt--again, a bit to salty for me. Then I used regular-grained sea salt--this tasted best to me).

-Bake for 17 minutes (they will look underdone, but they're not).

-cool slightly on sheet, then transfer to wire cooling rack.

-These are best eaten warm from the oven. When they are cooled and have been sitting around for a few hours, they get harder and harder--more like a store-bought cookie. Still yummy, but not as sublime as the just-out-of-the-oven cookies.