Mix dry ingredients. Add cacao powder, cornstarch, sea salt, and cinnamon to a small mixing bowl. Whisk all dry ingredients until there are no clumps and it’s thoroughly combined. Set aside.
Mix wet ingredients. Add milk, maple syrup, and egg yolks to a medium-sized mixing bowl. Whisk the mixture, being sure to break up the egg yolks completely.
Combine. Slowly add the milk mixture to the dry ingredients. Whisk ingredients together as you add, continuing to whisk until all ingredients are fully dissolved and well mixed. This step is crucial to make sure you end up with a nice, smooth consistency.
Heat. Pour the mixture into a medium-sized saucepan. Place over medium heat and continue whisking as it heats up. You want to be careful not to allow your mixture to burn or overcook. This is the purpose of consistently whisking. Cook over medium heat until your mixture thickens, about 4-5 minutes.
Add butter and vanilla. Once the mixture thickens, remove from the heat and add the butter and vanilla. Continue stirring until everything is combined and your mixture becomes a thick, pudding consistency.
Cover. Pour pudding into a bowl or container. Add a piece of plastic wrap over the top, allowing the plastic wrap to rest directly on top of the pudding to prevent any skin from forming.
Chill. Refrigerate the pudding for at least an hour before enjoying it.