Melt together the chocolate chips and butter. Let it cool to room temperature.
Meantime whip the cream. Take the chilled heavy whipping cream and vanilla in a bowl. Start whipping it using a stand mixer with a whisk attachment. Whip it until you get stiff peaks. Do not over-mix it otherwise cream will curdle and it will turn into butter.
Now check the melted chocolate is cooled to room temp. Add ⅓ part of the whipped cream. Gently fold in using a spatula. Do not mix it vigorously do it slowly and gently.
Now add another third portion of it and fold it.
Lastly, add the remaining ⅓ part and fold it in. You’ll notice how light and airy it is.
Divide the mousse into individual serving bowls/glasses. Cover it and keep it in the refrigerator to set for about 2 hours or more.