First I will mention that the liquid must be at least 160º but never exceed 185º or the eggs will solidify
Separate 4 eggs
– We will use the yolks in the recipe. You can reserve the whites for other uses
Mix egg yolks and 1/4 C granulated sugar together until the color lightens
Set aside
In a small, heavy saucepan, heat cream and vanilla until bubbles form at edges.
2 C heavy cream
1/3 cup castor (or white) sugar
1 tsp corn starch
1/2 vanilla bean (optional 1 tsp vanilla extract)
When cream is about 185º pour into your preheated double boiler – stirring almost constantly
Temper several ladles of hot liquid into the egg mixture, then slowly return to the double boiler, whisking constantly.
Continue to whisk until the sauce thickens. If a few clumps do develop, you can strain the liquid, but if the custard splits there is no remedy. You must start over.
Here is a video from Food Network about how to temper an egg mixture.
See also WikiPedia
