Add the eggs one at a time, beating vigorously with a wooden spoon or silicone spatula after each one. When the mixture looks thick, shiny and well blended, add the vanilla extract and the flour. Stir until you no longer see streaks of flour. Then beat vigorously (put those arm muscles to work!) for 50 strokes with the wooden spoon or spatula.
At this point, the mixture should be no more than slightly warm (if not, let it cool for a few more minutes). Fold in the chocolate chunks or chips.