Step 1Grease bottom and sides of 13x9-inch cake pan. Sprinkle with 2 tablespoons of the flour and shake and tilt pan to distribute. Tap out any excess.
Step 2Place butter and sugar in medium-size bowl and beat with an electric mixer on medium until well blended and smooth, 1 to 2 minutes. Scrape down sides of bowl with a rubber spatula a few times.
Step 3Add 1 egg at a time to batter, beating well after each until batter is smooth. Add lemon zest, lemon juice and vanilla and mix well.
Step 4Mix together flour, baking powder, baking soda, salt and poppy seeds in separate bowl. Add dry ingredients to butter mixture, alternating with buttermilk, in 3 batches, scraping down sides with rubber spatula after each addition. Beat until batter is smooth.
Step 5Pour batter into prepared cake pan. Use a spatula to spread batter evenly in pan. Bake at 350 degrees until toothpick inserted in center comes out clean, about 30 minutes. Let cake cool in pan 10 minutes before serving.