Melt the Chocolate: In a large microwave-safe bowl, combine the chocolate chips and ½ cup of heavy cream. Heat the mixture in 20-second increments, stirring well between each interval, until the chocolate is fully melted and smooth. Set aside and allow to cool to room temperature, stirring occasionally (about 15 minutes).
Whip the Cream: In a separate mixing bowl, using a hand mixer or stand mixer fitted with a whisk attachment, beat ⅔ cup of chilled heavy cream with the powdered sugar and vanilla extract until stiff peaks form. Once whipped, refrigerate the cream until the melted chocolate has cooled.
Combine Chocolate and Cream: Once the chocolate has cooled to room temperature, gently fold the whipped cream into the melted chocolate using a rubber spatula. Continue folding until the mixture is smooth and fully combined.
Chill and Serve: Transfer the mousse into individual serving dishes. Chill in the refrigerator for at least 2 hours to allow it to firm up. For a softer texture, let the mousse sit at room temperature for about 15 minutes before serving.