Separate the egg yolks and place them in a medium size bowl. Add the elderflower syrup and whisk into the yolks for a minute or so until the eggs are smooth. It’s fine to do this with a fork. Set aside.
Place the sugar and cornstarch in a medium size saucepan, and mix together with a whisk or fork.
With a whisk in hand, turn the heat to medium under the saucepan, and pour the half and half slowly and steadily into the dry ingredients while whisking constantly. Keep whisking until the mixture thickens and you see it begin to bubble. You may find this easiest to do in a double boiler. It will take a little longer, but it works well.
Remove from the heat momentarily, and pour 1/3 of the hot mixture into the eggs in the bowl. Whisk together in the bowl until it looks blended in. This won’t take long.
Pour the now hot egg mixture from the bowl, into the saucepan and return it to the stovetop. Turn the heat back to medium, and whisk the mixture constantly until you bring it to a boil. It will be thick and coat the bake of a spoon. Remove from the heat.
Turn the hot mixture into a bowl, and cover with parchment paper to prevent a skin from forming as it cools. Chill in fridge for at least 2 hours.