Mix together the chia seeds and oat milk in a large bowl and stir well. Allow to rest for 5 minutes, and then whisk again to prevent clumping.
Scoop out the solid white cream from the can of coconut milk (you should get about 1/2 cup, or slightly more, from a 13.5 ounce can of full-fat coconut milk). Reserve the liquid part for another use, such as smoothies or a curry.
Pour the chia pudding into a high-powered blender and add the coconut cream, cocoa powder, maple syrup, espresso, and vanilla. Blend until all of the ingredients are thoroughly combined and the mixture is thick, creamy, and fairly smooth.
Transfer the chia mousse to a jar and refrigerate for at least 1 hour. When ready to serve, add desired toppings and enjoy.