Mix the Dry Ingredients: In a medium bowl, whisk together the gluten-free 1:1 flour, unsweetened cocoa powder, baking soda, baking powder, and salt. Set aside.
Cream the Butter, Oil, and Sugars: In a large bowl, cream together the softened butter (or avocado oil) with granulated sugar and brown sugar until light and fluffy, about 3 minutes.
Add Wet Ingredients: Add the egg (or flax egg) and vanilla extract to the butter mixture. Beat until smooth and combined. Stir in the espresso powder until well incorporated.
Combine Wet and Dry Ingredients: Gradually add the dry ingredients to the wet mixture, mixing until just combined. Be careful not to overmix to keep the cookies tender.
Fold in the Chocolate: Gently fold in the chopped dark chocolate or chocolate chips. The chocolate will melt slightly while baking, creating pockets of gooey chocolate goodness.
Scoop the Dough: Using a cookie scoop or tablespoon, drop rounded balls of dough onto the prepared baking sheet, spacing them about 2 inches apart to allow room for spreading.
Refrigerate dough: While you can bake right away, this step makes all the difference in taste and texture. I like to rest in the fridge for at least 12 hours, ideally 24-36 hours.
Bake the Cookies: Bake at 350 for 9-10 minutes, or until the edges are firm but the centers are still slightly soft. Let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.