In a heavy saucepan, bring the half & half and lavender just to a boil. Turn off heat and let sit for 20 minutes to allow lavender to infuse into the heavy cream. Strain out lavender.
Bring back to a boil and turn off the heat.
Whisk the egg yolks and sugar together in a mixing bowl.
Whisk in 1/3 of the hot heavy cream into the egg/sugar mixture to allow the temperature of the eggs/sugar to equal that of the heavy cream.**
Return the entire mixture to the saucepan with the remaining heavy cream.
Whisk the sauce over very low heat, stirring constantly with a wooden spoon or rubber spatula until it is thick enough to coat the back of a spoon; this will happen quickly and remember that the liquid is hot and will be much thicker when it cools.
Do not allow the mixture to curdle.
As soon as the sauce thickens, removes it from heat and pour it through a fine mesh strainer into a clean bowl.
Chill the sauce over an ice bath (bowl with ice underneath).
Cover and refrigerate.