Start by dissolving the instant coffee in hot water. Stir until the coffee granules are completely dissolved. Set this mixture aside to cool.
In a mixing bowl, pour in the cold heavy whipping cream. Using an electric mixer, whip the cream on medium speed until soft peaks form. Gradually add in the powdered sugar while continuing to whip until stiff peaks are achieved. Be careful not to over-whip, as this can result in a grainy texture.
In another bowl, combine the mascarpone cheese (or cream cheese) with the vanilla extract and the cooled coffee mixture. Mix until smooth and well-combined.
Gently fold the whipped cream into the mascarpone mixture using a spatula. Be careful to maintain the light and airy texture of the whipped cream. This step is crucial for achieving that mousse-like consistency.
Sift the unsweetened cocoa powder over the mousse and fold it in until fully incorporated. This step enhances the chocolate flavor and gives your mousse a beautiful dark hue.
Spoon the mousse into individual serving cups or bowls. For an elegant touch, you can garnish with grated semi-sweet baking chocolate on top. Refrigerate for at least 2 hours to allow the flavors to meld and the mousse to set.