12 ounces (2 cups) good-quality bittersweet chocolate chips or féves***
In a the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl using a handheld electric beater, beat together butter and brown sugar until light and creamy. Scraping down the bowl and beater as needed, beat in the egg until fully incorporated. Beat in 2 cups (6 1/2 ounces) of oats, again scraping down as needed.
Sprinkle the flour, baking soda, and salt over the wet ingredients. Drizzle the vanilla on top. Fold in with a spatula just until mostly incorporated. Add the remaining 1 cup (3 1/4 ounces) oats and the chocolate chips, and fold in just until evenly dispersed into the dough and all ingredients are fully incorporated.
Scoop the dough into 2 1/4 ounce portions (about 1/4 cup each), and roll into balls. These *can* be baked immediately (jump to step 5), preferably only if using a silicone baking mat, but are best if chilled for at least a couple hours.
To chill or freeze before baking: Transfer cookie dough balls to a baking tray and freeze for 1 hour. Transfer to a zip-top plastic bag, seal, and store in freezer until ready to bake.
To bake cookies: Preheat oven to 350ºF with oven racks in the top and bottom-third positions. Line two cookie sheets with parchment paper or silicone baking mats. While oven is preheating, arrange cookie dough balls on prepared baking sheets, no more than 9 cookies to a sheet. Bake in preheated oven for 14 minutes (or 16 minutes if frozen), switching the baking sheets on the racks halfway through. Cookies are done when puffed up and golden.
Allow cookies to cool briefly on baking sheets before transferring to a wire rack to cool completely. These can be stored in an airtight zip-top bag at room temperature for 1-2 days, or in the refrigerator for about 5 days, but are best served at room temperature. They can also be stored in the freezer for longer periods.
*Does not include (optional) chilling time
**You can sub regular (non gluten-free) oats and flour if gluten isn’t an issue