Beat butter and brown sugar until lighter. Add the egg and vanilla. In another bowl combine flour, cornstarch, baking soda, baking powder, xanthan and salt; mix into the butter mixture.

Chop the chocolate fairly small so it does not obstruct shaping. Mix most into the dough, reserving a little for the tops. Cover and refrigerate 20–30 minutes, until less sticky but still scoopable.

For each stick, place about 2 tbsp dough on parchment and fold the paper over it. Hold the lower sheet steady and use a bench scraper against the folded paper to gently elongate the dough into a 6–7-inch / 15–18 cm cylinder. Make 18. Cover and freeze at least 30 minutes.

Heat a conventional oven to 350°F / 180°C. Place the frozen sticks on lined trays at least 1 3/4 inches apart. Bake 8 minutes, then use two straight-edged scrapers or butter knives to gently straighten their sides. Add the reserved chocolate.

Bake another 4–5 minutes, until golden with deeper-colored edges. Cool 5–10 minutes on the tray before transferring to a rack.