In a chilled bowl, beat heavy whipping cream with powdered sugar until soft peaks form. Slowly drizzle in the cold milk and whip until combined.
Add the instant chocolate pudding mix and continue beating until the mixture is thick and smooth.
Fill each chocolate cup or dessert glass with the mousse using a spoon or piping bag. Refrigerate for at least 15 minutes before serving. Garnish with chocolate shavings or sprinkles.