Pour about half a cup of the hot milk/cream mixture in a thin stream, very slowly into the bowl containing the egg yolk mixture, while you quickly whisk the mixture with a wire whisk. You do this to make sure you do not scramble and curd the egg yolks. If the temperature is too high or the mixture is not mixed together quickly enough, the mixture will curd. In that case, discard and start over.
Return the egg and milk mixture to the pan, and return to the heat. Using a rubber spatula, stir constantly in a figure 8 fashion over the bottom and the edges of the pan, making sure you scrape the bottom well. Do not pause, but continue.
Bring the mixture to a simmer, until the mixture reaches a temperature of between 170-175oF, or 76-80oC. This will take around 2 or 3 minutes. When the mixture reaches that temperature, remove from the heat. Do not heat above 180oF/82oC, or the mixture will curdle. The sauce will not thicken a lot, beyond the line-on-the-spoon stage, so don't be alarmed when it is still a bit runny.