In a large bowl, whisk together the gluten-free flour, xanthan gum, cornstarch, sugar, and salt until well combined.
Create a well in the center of the dry ingredients and pour in the lukewarm water and melted butter. Using a sturdy spoon or your hands, gradually incorporate the dry ingredients into the wet ingredients until a shaggy dough forms.
Turn the dough onto a lightly floured surface and knead for about 5 minutes until smooth and pliable. If the dough seems too sticky, add a little more gluten-free flour, 1 tablespoon at a time.
Cover the dough with plastic wrap or a damp towel and let it rest at room temperature for 30 minutes.
After resting, divide the dough into 4 equal portions. On a lightly floured surface or parchment paper, roll out each portion into a rectangular shape, about 1/8 inch thick.
Using a sharp knife or a pizza cutter, cut the dough into rectangles, approximately 10x8-inch. Cut into smaller rectangles as desired. Cover them with a damp towel or plastic wrap to prevent drying out.
Use the gluten-free phyllo dough sheets immediately or store them in an airtight container in the refrigerator for up to 3 days.
Notes
If possible, use a thin rolling pin when rolling out the dough, and don't apply too much pressure. Gently roll the dough from the center outwards in each direction.
Cover the dough with a damp kitchen towel when not using to keep it from drying out.
To Store: Wrap in plastic wrap and refrigerate for up to 3 days.
To Freeze: Layer parchment paper or plastic wrap between each sheet and roll tightly before wrapping in plastic. Freeze for up to 3 months.