1. In a small pot over medium low-heat, combine the butter and tahini. Cook until the butter melts and the tahini is softened, 3 to 4 minutes.
2. In a large bowl, combine the melted butter and tahini, ½ cup of the granulated sugar, and the brown sugar. Beat with a whisk until fully incorporated.
3. Add the egg and vanilla and whisk until combined and light in color, 1 minute.
4. Add the gluten free flour, salt, baking soda, and xanthan gum. Fold together with a rubber spatula until combined.
5. Cover and refrigerate until the dough is firm, 2 hours and up to overnight. 
6. Preheat the oven to 350º. Line 2 sheet pans with parchment paper. In a small bowl, whisk together the remaining 2 tablespoons granulated sugar and the sesame seeds. 
7. Portion the dough into 1-tablespoon-size balls. Use your thumb to make a large indent in the dough balls, add ½ teaspoon of guava paste to each well, and carefully form the dough around the paste. 
8. Roll the dough balls between your palms to smooth them, then place them in the sesame sugar, turning to coat. Place the cookies on the baking sheet, about 2” apart. 
9. Bake for 12 to 15 minutes until the cookies are just set. Let cool for 3 minutes, transfer to a wire rack, and let cool completely. 
10. Store leftover cookies in an airtight container for up to 5 days.

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