Single Serving Method:

1. Mix dry ingredients: In a microwave-safe mug or bowl, whisk together sugar, cocoa powder, cornstarch, and salt until well combined.
2. Add milk gradually: Slowly add milk, whisking constantly to prevent lumps from forming.
3. First microwave: Microwave on high for 1 minute.
4. Stir and continue: Remove and stir well. The mixture will still be thin.
5. Second microwave: Microwave for another 30-60 seconds, watching carefully to prevent overflow.
6. Check consistency: Stir again. If not thick enough, microwave in 15-second intervals until desired thickness is reached.
7. Add finishing touches: Stir in butter and vanilla extract until smooth. Add chocolate chips if using.
8. Serve: Enjoy warm or cover with plastic wrap (touching the surface to prevent skin forming) and refrigerate.

Multiple Servings Method:

1. Combine dry ingredients: In a large microwave-safe bowl, whisk together sugar, cocoa powder, cornstarch, and salt.
2. Add milk: Gradually whisk in milk until smooth and no lumps remain.
3. Initial cooking: Microwave on high for 2 minutes.
4. Stir and continue: Remove and stir thoroughly. Mixture will be thin.
5. Continue cooking: Microwave for 1-2 more minutes, stirring every 30 seconds.
6. Final cooking: Continue microwaving in 30-second intervals, stirring between each, until pudding coats the back of a spoon.
7. Finish: Stir in butter and vanilla until smooth. Add chocolate chips if desired.
8. Portion and serve: Divide into serving cups or bowls.