Step 1. Preheat and Prep: Preheat your oven to 350°F and line large baking sheets with parchment paper.

Step 2. Melt Peanut Butter Mixture: In a small saucepan over medium-low heat, melt the peanut butter and chopped butter together, stirring until smooth. Set aside to cool slightly.

Step 3. Combine Dry Ingredients: In a large bowl, whisk together the gluten free flour, xanthan gum, baking soda, salt, and granulated sugar. Add the light brown sugar and whisk again, breaking up any lumps.

Step 4. Mix the Dough: Create a well in the dry ingredients and pour in the melted peanut butter mixture. Mix to combine. Add the beaten eggs and vanilla, and mix again until a soft, sticky dough forms.

Step 5. Shape the Cookies: Scoop dough by the 2-tablespoonful and roll into balls. Place on the prepared baking sheets about 1 1/2-inches apart and press into a 1/2-inch thick disk.

Step 6. Create Crisscross Pattern: Sprinkle the tops of the cookies with optional sugar. Press the tines of a wet fork into the top of each cookie, then press again in the opposite direction to create the classic pattern.

Step 7. Bake: Bake one sheet at a time for 15 minutes for softer cookies, or 18 minutes for crunchier cookies, until pale golden.

Step 8. Cool Completely: This is a crucial step! Let the cookies cool on the baking sheet for at least 5 minutes until set, as they are very fragile when hot. Transfer to a wire rack to cool completely.