Pour the cream into a saucepan, then add the sugar and the elderflower heads, previously checked to avoid unwanted guests.
Bring the cream to a gentle boil, stirring occasionally to melt the sugar, then turn off the heat and let it cool down.
Slightly beat the egg whites with a fork until you see some bubbles on the surface, then sieve the cream and pour it onto the egg whites. Stir just enough to mix the ingredients.
Heat the oven to 120°C.
Melt 100 g of sugar in a non-stick pan until it becomes an amber caramel.
Pour the caramel into a loaf pan, wait a few seconds until the caramel sets, then pour in the cream, too.
Place the panna cotta mould in a larger pan filled with water and bake on the bottom rack for about 1 hour and 20 minutes, or until it is elastic and compact to the touch. Be careful not to burn yourself when checking the panna cotta!
Remove the panna cotta from the oven, let it cool down, then stash in the fridge for a few hours, or, even better, until the next day, so that it will be completely set and the caramel will turn into a sauce.
When serving the panna cotta, dip the pan for a few seconds in a bowl of boiling water, then unmould into a plate. The caramel will run everywhere, so choose a plate which will easily keep the sauce, you don’t want to spill it, it is so good.