In a large saucepan, heat the cream and milk until beginning to steam. Set a heatproof medium bowl in a large bowl of ice. In another bowl, whisk the egg yolks with the elderflower concentrate until smooth. Gradually whisk in the hot cream.
Return the mixture to the saucepan and cook over moderate heat, stirring gently with a wooden spoon, until the custard coats the back of the spoon, about 6 minutes; do not let it boil. Strain the custard into the bowl in the ice bath and let cool completely, stirring often.
The sauce can be refrigerated for up to 1 day.
Elderflower concentrate, made in England from elderflowers and lemons, is available by mail from Mushrooms & More, 914-682-7288.