In a small bowl, combine the instant coffee with the hot water. Stir until the coffee is completely dissolved. Set aside to cool slightly.
In a large mixing bowl, pour in the cold heavy whipping cream. Using an electric mixer, whip on medium-high speed until soft peaks form. Gradually add the powdered sugar, continuing to whip until stiff peaks form.
In another bowl, combine the mascarpone cheese and vanilla extract. Using a spatula, mix until smooth and creamy.
Gently fold the whipped cream into the mascarpone mixture using a spatula. Be careful not to deflate the whipped cream; you want to maintain that fluffy texture.
Once combined, add the cooled coffee to the mixture and fold gently until just blended.
Spoon the mousse into serving glasses or bowls and refrigerate for at least 2 hours, or until set.
Before serving, dust the tops with unsweetened cocoa powder and grate semi-sweet baking chocolate over the mousse for a finishing touch.
