Combine all ingredients but vanilla extract and peanut butter (if using) in a medium-sized saucepan. Whisk until the cocoa powder and the cornstarch dissolve entirely. This step is essential to avoid lumps in the preparation.
Then, turn on the heat and bring the preparation to a boil over medium heat while continuously stirring.
When the mixture in the saucepan is boiling, lower the heat to a minimum. Continue to cook over low heat, constantly whisking for 1 minute until thickening.
Remove from the heat, add the vanilla extract and the peanut butter if using, and combine. Pour into individual serving dishes/ramekins, let them cool, and cover with plastic wrap to avoid skin from forming, and refrigerate a couple of hours until set.
Serve topped with shaved chocolate, shredded coconut, fresh fruits, or cashew cream! Enjoy!