Pour half and half into a medium saucepan off heat, sprinkle gelatin over and let stand 5 minutes. Set saucepan over medium-low heat, whisking constantly until gelatin dissolves, about 3 minutes (do not boil).
Increase heat to medium and add 1 cup of heavy cream and sugar to sauce pan, continue whisking until sugar has dissolved and mixture is steaming hot, about 5 minutes (do not boil). Remove from heat and add vanilla and chopped chocolate all at once; whisk until chocolate has melted and mixture is completely smooth.
Either transfer mixture to a glass measuring pitcher for easy pouring or carefully pour directly from the saucepan into glass tumblers or ramekins (see recipe note #2). Refrigerate until completely set, 4 hours or overnight. (Cover if refrigerating overnight)
Just before serving, add ½ cup cold heavy cream, sugar and vanilla to a medium bowl. With an electric mixer, beat on high speed until medium stiff peaks form. (You can also do this by hand by vigorously whisking with a wire whisk.)
Top each serving of chocolate panna cotta with a dollop of whipped cream and sprinkle chocolate shavings over if desired. Serve immediately.
