Whisk the dry ingredients. In a medium-sized bowl whisk together the gluten-free flour, Dutch-processed cocoa powder, espresso powder, baking soda, baking powder, and salt. Set the bowl aside.
Cream the butter and brown sugar. In a large mixing bowl or stand mixer, add the softened butter and brown sugar. Cream together until light and fluffy, around 3-4 minutes, scraping down the sides of the bowl.
Add the egg, egg yolk, and vanilla extract. Continue to beat until combined.
Pour in the dry ingredients. With the mixer on low, slowly add the flour mixture to the butter mixture. Mix until just combined.
Prepare baking sheets. Preheat the oven to 350° F. Line two large baking sheets with parchment paper. Use a 1-ounce cookie scoop to scoop the cookie dough out into 30-gram balls. Roll the dough with clean palms, and place each ball of dough 2 inches apart on the prepared sheet.
Bake the cookies. Place one baking sheet at a time in the oven for 8 minutes. The edges will be crisp and the middle will look slightly underdone.
For bakery-style cookies. Right out of the oven, place a large cookie cutter or round glass over the top of the freshly baked cookies, and swirl them in a circular motion rounding them into perfect circles. Sprinkle the cookies with flaky sea salt and allow them to rest on the baking sheet for 5 minutes, then transfer to a cookie rack for 1 hour to cool before storing.