Preheat the oven to 325°F. Line a baking pan with parchment paper or silicone baking mat.
Beat butter and sugar for about 3 minutes, until pale and fluffy. Scrape the sides of the bowl. Add egg and vanilla and beat until combined.
Sift and add all dry ingredients and melted chocolate and beat until combined. Add chopped chocolate.
Roll the dough into flat rounds. If the mixture gets too sticky, refrigerate for 10 minutes and roll smoothly between hands.
Bake for 15 minutes or until cracks form. Let cool on tray, then drizzle with chocolate.
Notes
Tip: Flatten the cookie dough when rolling to create more surface area for all of that important chocolate drizzle. Feel free to swap your dark chocolate chunks with milk or white for a little variety.