Orange zest and juice lend this simple vanilla cake bright flavor notes, while poppy seeds give it wonderful crunch. Be sure to buy only pure vanilla extract with no artificial flavors added (check the label), as it is the predominant flavoring of the dessert that's enhanced by mixing vanilla sugar into the batter before baking and sprinkling more vanilla sugar over the cake, along with more poppy seeds, as soon as it comes out of the oven.