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 1/2 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.
Video
Notes
Use good quality bar chocolate for this recipe. Do not use chocolate chips, they are made to hold their shape and won't melt properly leaving you with gritty panna cotta. I used 70% dark chocolate, but you can use any good quality chocolate you like!
If you prefer to turn these out onto serving plates, I recommend using 4 ounce / 1/2 cup ramekins instead of tumblers. Lightly oil the ramekins with a flavorless oil before pouring in warm chocolate panna cotta mixture. When the desserts are set and ready to serve, run a thin knife around the edge of ramekins. Dip each ramekin individually in a bowl of hot water for 5 seconds (taking care not to let any spill over into your panna cotta). Invert a small serving plate over the center of the ramekin. Hold plate and ramekin firmly and turn upside down (so the ramekin is on top). Wiggle the ramekin back and forth to release the panna cotta and lift the ramekin away. Repeat with remaining ramekins. Serve.
For those outside the US who may not have half and half in your grocery stores, substitute with 1/2 heavy cream and 1/2 whole milk. So for this recipe with 1 1/2 cups half and half, you would use 3/4 each of heavy cream and whole milk.