Cream the softened butter and brown sugar until pale and fluffy. Add the egg and vanilla and mix until smooth.
In a separate bowl, mix the gluten-free flour, cornstarch, baking soda, baking powder, xanthan gum and salt. Add to the wet ingredients and mix into a smooth dough.
Chop the chocolate into small pieces. Mix most of it into the dough, reserving a little for topping.
Chill the dough for 20–30 minutes, until firmer but still easy to shape.
Use 2 tbsp dough per cookie and shape into sticks about 15–18 cm long. Place on a lined baking sheet.
Cover and freeze for at least 30 minutes.
Preheat the oven to 180°C and line a baking sheet with parchment paper.
Place the frozen cookie sticks at least 4.5 cm apart and bake for 8 minutes.
Remove from the oven and gently push the sides back into a neat stick shape using two bench scrapers or knives. Add the reserved chocolate on top.
Bake for another 4–5 minutes, until golden with slightly darker edges.
Cool on the baking sheet for 5–10 minutes, then transfer to a wire rack to cool completely.