In a small bowl, whisk the cocoa powder into the warm water until it dissolves completely. Set it aside. In a small, heat-safe bowl, place the chopped chocolate, and place the bowl over a small saucepan of simmering water, making sure the bowl does not touch the water. Melt the chocolate over the simmering water, stirring occasionally, until smooth. (Or, you can melt the chocolate in the microwave in short, 30-second bursts, stirring well in between.) Remove the melted chocolate from the heat and add the Nutella and the cocoa powder dissolved in water into it, whisking until smooth. The mixture should be relatively thick but smooth and shiny. Set the bowl aside.
In the bowl of a stand mixer fitted with the whisk attachment, or in a large bowl with a hand mixer, whip the heavy whipping cream on medium-high speed until stiff, but not dry, peaks form. Pour the chocolate mixture into the bowl of whipped cream slowly, carefully folding the ingredients together and taking care not to deflate the whipped cream more than necessary. Fold until no white streaks remain.
Transfer the mousse to a large pastry bag fitted with a large, plain, open piping tip. Pipe the mousse into small ramekins, jars or a well-greased miniature cheesecake pan. Or just spoon it in - maybe not as pretty but it does the job. Place in the refrigerator to chill until firm (about 2 hours). Serve chilled, with the (optional) whipped cream and chocolate shavings.