Buy the form the recipe needs
For a bar, note the type and cacao percentage if the recipe gives it. A very sweet milk chocolate and a dark bar will change the balance of a cookie differently. Keep the wrapper until the bake is finished so your notes can identify the product.
Check natural or Dutch-process cocoa
Dutch-process cocoa has been alkalized, making it different in acidity from natural cocoa. King Arthur's baking guidance explains why that matters in some recipes, especially where cocoa interacts with baking soda. A recipe that names one type is giving a useful technical instruction.
Do not adjust leavening by a general rule copied from a different cake. Follow substitution guidance for the particular formula when available. If the recipe allows either type, choose by the intended flavor and color, and record which you used.
Make pieces that suit the bake
A large chocolate chunk can create a concentrated pocket in a cookie. Finely chopped chocolate spreads through more of the mixture. Cut to the recipe's approximate size and include the smaller pieces if it calls for them, instead of keeping only the most photogenic chunks.
For melting, use the recipe's gentle heating method and stir as directed. Keep water out unless the preparation deliberately includes it. A bowl that is overheated while you answer a message can cause more trouble than the difference between two reasonable brands.
Store for the next batch
Keep chocolate and cocoa according to their labels, protecting them from heat, moisture, and strong odors. Buy a useful amount rather than a large package that has no next recipe. Mark the cocoa container clearly so it cannot be confused with another brown powder.
After baking, note the sweetness, texture, and distribution of chocolate rather than only whether the result was rich. If you change products, keep the rest of the formula steady. That makes the next comparison useful instead of turning it into a completely different cookie.
Keep in mind
Check milk, soy, nut, and other allergen statements on the particular chocolate product.
A chocolate percentage alone does not describe every detail of flavor or baking behavior.
