Put the coconut oil or butter in a microwave-safe bowl, cover, and microwave on medium power until melted. (Alternatively, melt in a small saucepan.) Cool slightly. Whisk the light brown sugar, eggs, butter, salt, and both types of vanilla in a large bowl until smooth.

Whisk the flour, and baking soda, in another bowl. Stir the dry ingredients into the wet ingredients with a wooden spoon; take care not to over-mix. Finally, stir in the bittersweet chocolate chips and walnuts, if using; cover it tightly, and chill in the refrigerator for at least 30 minutes.

If chilling for 3+ hours, make sure you let the cookie dough sit at room temperature for at least 1o to 15 minutes before rolling into balls; the cookie dough will be very stiff after being in the fridge that long.

Form and bake the cookies: Preheat the oven to 350 °F. Position racks in the upper and lower thirds of the oven. Line two baking sheets with parchment paper; set aside. If you only have 1 baking sheet, let it cool completely between batches.

Scoop two heaping tablespoons of the dough onto the prepared pans. Roll each mound in your hands to form a ball.

Place on the prepared baking sheet, about 2 to 3 inches apart. Bake one sheet at a time until golden brown around the edges but still light in the center, about 10 to 11 minutes. Remove from the oven and let cool slightly on the baking sheet, then transfer the cookies to a wire rack to cool completely. Repeat forming the remaining dough into balls. Store cookies in a tightly sealed container for up to 3 days.