Preheat oven to 190°C (or 170°C fan bake). Set aside an oven-proof frypan or a baking dish (measuring about 15-20cm in diameter). Melt butter. Place cookie mix into a large bowl, add melted butter and milk. Mix gently to combine. Transfer mixture to prepared pan/dish and spread out evenly with the back of a spoon. Sprinkle over chocolate drops.