Tempering Yolks
Beat the egg yolks in a heatproof bowl. Slowly stir in some of the hot chocolate mixture. This gently heats the yolks, or "tempers" them, so they won’t curdle when added to the pastry cream.
Covering Surface
To prevent a skin from forming on the cooked pastry cream as it cools, press plastic wrap directly onto the surface; let cool without stirring.