Preheat oven to 350 degrees F and grease and flour an 8 by 12 inch cake pan, knocking out excess flour.
In a bowl whisk together flour, sugar, cornstarch, poppyseeds, baking powder, and salt and with fingers blend in butter until incorporated. In a large measuring cup, lightly beat together milk, zest, vanilla and egg.
Stir milk mixture into flour mixture until just blended and pour batter into pan. Bake cake for 35 minutes or until tester comes out clean. Remove cake from pan and cool on rack.