• Place cream and sugar in a small saucepan.
• Split the vanilla bean in half lengthwise with a sharp knife, and scrape out the seeds with a knife; add the seeds and pod to the saucepan.
• Bring to a simmer over low heat, stirring occasionally. (Just a simmer, not a boil.)
• Discard the vanilla pod.
• While the cream is heating, “bloom” the gelatin: sprinkle over the milk in a small bowl and let stand until the gelatin softens, 5 - 10 minutes.
• Pour the gelatin mixture into the hot cream mixture (no need to scrape the extra gelatin bits left in the bowl) and whisk until dissolved. (Add vanilla extract, if using.) Remove from heat. Whisk and make sure that the sugar and gelatin have fully dissolved; then stir in the yogurt. (If the yogurt has become watery, pour off the water before incorporating.)
• Pour through a fine sieve into a large bowl, to remove vanilla debris and make it smooth. Place in an ice bath or refrigerate for about 15 minutes until cooled, whisking now and again.
• Note: if you pour it straight into ramekins or serving dishes, the vanilla seeds might sink to the bottom, or the yogurt might split.
• Spray ramekins with neutral tasting cooking oil spray. (If you’re serving in the dishes this step isn’t necessary.)
• Ladle into six to eight 4-ounce ramekins, cover with plastic wrap and refrigerate until set, at least 6 hours or preferably overnight.

Make the Sauce

• Heat the Nutella, heavy cream, and milk over low heat, stirring often. (I use a double boiler) Set aside in a warm place if not using immediately. Pour into a squeeze bottle for easy drizzling.

To Unmold

• Run a sharp knife around the outside edge of the panna cotta to loosen it. Set each ramekin three-quarters of the way in hot water for 10 seconds. Place a small serving plate on top and flip over to unmold. If it doesn’t come out, put it back in the water for a few more seconds and try again. This can be a little tricky because you don't want to leave it in the water for too long because it will start to melt. If it doesn't come out easily, tap it on the counter or rub in between your hands to warm it up. It will come out eventually!

To Serve

• Drizzle or ladle with warm sauce, or garnish with strawberry slices.