Scrape the vanilla pod out with a blunt knife. Place the gelatin leaf in plenty of cold water. Put three quarters of the cream in a pan with the sugar, the vanilla pod and the vanilla marrow. While stirring, bring to a boil over medium heat. Let 30% of the boil down, this means that after boiling, there is 30% less weight in the pan. So occasionally weigh while cooking. Write down the weight of the pan at the beginning and weigh the pan with the cream. Also write down the weight of the cream. A quick calculation, but it is for a good cause, namely a divine panna cotta. You can also look closely with some experience. When air bubbles bubble up slowly, the cream is sufficiently reduced and nicely thick.