1. Sift flours, xanthan gum, baking soda, baking powder and salt into a bowl. Set aside.
2. Using a mixer fitted with paddle attachment, cream butter and sugars together until very light, about 5 minutes. Add eggs, one at a time, mixing well after each addition. Stir in the vanilla. Reduce speed to low, add dry ingredients and mix until just combined, 5 to 10 seconds. Drop chocolate pieces in and incorporate them without breaking them.
3.* Press plastic wrap against dough or place in a large ziplock bag and refrigerate for 24 to 36 hours. Dough may be used in batches, and can be refrigerated for up to 72 hours.
4. You can cook the dough right away - just skip to step 5.
5. When ready to bake, preheat oven to 350°F. Line a baking sheet with parchment paper or a nonstick baking mat.
6. Scoop 2-inch balls of dough onto baking sheet, making sure to turn horizontally any chocolate pieces that are poking up; it will make for a more attractive cookie.
7. Press down gently on the top of the cookies to create a flatter surface.
8. Sprinkle lightly with sea salt and bake until golden brown but still soft, about 20-22 minutes.
9. Transfer sheet to a wire rack for 10 minutes.
10. Repeat with remaining dough. Or you can reserve dough (refrigerate) for baking remaining batches the next day. Eat warm!