In a medium-sized bowl, whisk together the egg yolks, sugar and cornstarch until the mixture is smooth.
Heat the milk, cream and vanilla in a small saucepan just until bubbles form along the edges of the pan.
Remove 1/2 cup of the hot cream and whisk it into the egg mixture, whisking constantly. Gradually pour the egg yolk mixture back into the saucepan with the rest of the cream mixture, whisking constantly to prevent clumping. Continue to cook, whisking constantly, until the mixture leaves a path on the back of spoon when you draw your finger across it. DO NOT BOIL.
Cover and chill. Can be made a day in advance.
Adjust the thickness of the sauce to serve your purpose by adding more or less cornstarch.