Chinese Chili Flakes are dried chilies that have been crushed into flakes. Look for them at the Asian grocery store. You’ll find many different brands and forms of dried chilies: powder, whole, chopped, medium flake, large flake, etc. For this recipe, look for medium flake crushed chilies with seeds. They should be vibrantly red and aromatic.
Please note: Chinese Chili Flakes are different from Italian dried chili flakes (the kind you sprinkle over pizza). By contrast, Italian chili flakes are darker, less aromatic, and slightly bitter. If you substitute with Italian chili flakes (or cayenne or paprika), it will not taste the same.