First, add 2 cups water to your Instant Pot. Place a large oven-safe bowl over the top of the pressure cooker and press the sauté button. Let the water come to a boil.
When the water is hot, melt the butter in the bowl. Then mix in the egg yolk, sweetener, and vanilla. Continuously stir the egg mixture until it thickens. When the egg mixture has thickened add the chocolate chips and continue to stir until smooth. Set the bowl to the side until ready to use.
Add the heavy cream to a bowl and whip it with an electric hand mixer until the cream is firm and forms stiff peaks. Then fold the whipped cream into the chocolate.
Pour the mousse into ramekins or bowls and then let the mousse firm in the refrigerator for at least 30 minutes before garnishing and serving. If the mousse gets too firm, let it soften on the counter.