Brush six 3/4-cup custard cups very lightly with canola oil; set aside. 

Pour milk into a medium bowl. 

Sprinkle gelatin over milk and let stand until gelatin softens, about 5 minutes. 

Meanwhile, stir heavy cream and sugar together in a heavy medium saucepan over medium high heat until sugar dissolves, about 2 minutes. 

Bring to a boil, then remove from heat, add chocolate and whisk until melted. 

Whisk warm chocolate mixture into milk mixture in bowl and stir until completely dissolved. 

Pour into cups, cover and refrigerate for at least 6 hours. 

When ready to serve, run a knife around the inside edge of the cups to loosen panna cotta, then turn them out onto plates and garnish with oranges and pistachios.