Pour milk into a medium saucepan. Sprinkle powdered gelatin over the milk. Wait 5 minutes to allow it to "bloom". (Do not heat yet.)
Meanwhile, melt chocolate chips in the microwave in 30 second intervals at 50% power, stirring between each interval. Stir in Nutella until combined. Set aside.
Once the gelatin has bloomed, heat the milk to medium heat until just simmering. Whisk continuously to dissolve the gelatin. Remove from heat.
Add the melted chocolate and Nutella mixture and whisk again until combined. Allow the mixture to cool for 1 minute.
Whisk in vanilla extract and Greek yogurt until smooth. Let the mixture cool to room temperature.
Spray your molds or glasses with cooking spray. Then, wipe them out with a paper towel (leave just a light amount of coating on them). Pour the panna cotta mixture into your molds, dividing it equally.
Cover with plastic wrap and chill in the refrigerator for at least 2 hours (preferably 4 hours), until set.
Remove the panna cotta from the molds*, add desired toppings, and serve!