Make sure to have refrigerated the can of full-fat coconut milk overnight.
Extract only the solids of the coconut milk can and place them in a small saucepan. Bring the saucepan to a simmer. Let it melt, stirring until smooth.
Combine the creamy oat milk, cornstarch, and icing sugar, in cold, until smooth. Pour the mixture into the simmering coconut milk, and bring to a simmer again over medium heat. Stir in the pure vanilla extract and the cacao butter drops if adding, stirring for a few minutes, until the cacao butter has melted and the mixture has thickened. Let simmer until it becomes thick and creamy, stirring continuously.
Turn off the heat and allow the mixture to get to room temperature. This is a very important step!
Once the vanilla mixture is room temperature, place the aquafaba and cream of tartar in a large mixing bowl and, using an electric mixer with beaters, whip on medium speed; turn to high speed. Beat until white and glossy stiff peaks form. Make sure you have achieved stiff peaks. The best way to do so is by inverting the bowl with the whipped aquafaba. If it slides down at all, you need to keep whipping until it doesn't move at all. Stir in the granulated sugar and vanilla extract, while whipping at high speed. Keep whipping for 10-15 more minutes until it gets thicker and looks completely smooth (without any sugar pieces left).
Now whip the vanilla mixture inside the saucepan until it's completely smooth and creamy.
Gently and gradually fold the vanilla mixture from the saucepan inside the bowl. Use a silicone spatula and a gentle folding motion. Don't whip them together, only use a silicone spatula!
Place the vanilla mousse into glasses.
Refrigerate for a few hours (around 4 hours), or until the vanilla mousse has set. Serve chilled and enjoy this fancy and delicious dessert!