Combine the instant coffee powder and hot water until the coffee dissolves. Set aside. Heat a saucepan over medium-low heat. Add the butter and cook, stirring, until the butter melts. Stir in the cocoa powder and dissolved coffee and mix until smooth. Stir in the sweetened condensed milk and stir until well mixed. Remove the pan from the heat and set aside until cool. When the chocolate mixture is cool, add the whipping cream to a large, chilled bowl and beat with an electric hand mixer until stiff peaks form. Add the chocolate mixture to the whipped cream and fold gently to combine. Spoon the mousse into individual dessert dishes and chill for 2 hours or until set. Garnish as desired before serving with chocolate shavings, whipped cream, or fresh fruit, if desired.