Grease a 4-cup (8x4-inch) or six-cup loaf pan, line the bottom with parchment or waxed paper, and grease and flour the paper. Or grease and flour a fluted tube pan.
Lightly combine the milk, eggs, and vanilla in a medium-size bowl.
In a large mixing bowl, combine the cake flour, sugar, baking powder, and salt. Add the lemon zest and poppy seeds.
Mix on low speed for 30 seconds to blend.
Then add the butter and half the egg mixture. Mix on low speed until the dry ingredients are moistened.
Increase the speed to medium (high if you are using a hand mixer), and beat for 1 minute. Scrape down the sides of the bowl.
Gradually add the remaining egg mixture in two batches, beating for 20 seconds after each addition, and scraping down the sides of the bowl as necessary.
Spoon the batter into the prepared pan, and smooth the surface with a spatula. The batter will be almost 1/2 inch from the top of a 4-cup loaf pan.
Bake, covering the pan loosely with buttered aluminum foil after 30 minutes to prevent over browning, until a toothpick inserted into the center comes out clean, 55 to 65 minutes (35 to 45 minutes in a fluted pan). To get an attractive split down the middle of the crust, wait until the natural split is about to develop (when cake has cooked for about 20 minutes), and then use a lightly greased sharp knife to make a shallow mark about 6 inches long down the middle of the cake. (This must be done very quickly so that the oven door does not remain open very long, or the cake will fall). When the cake splits, it will open along the mark.
Shortly before the cake is done, prepare the Lemon Syrup: In a small pan over medium heat, stir the sugar and lemon juice together until the sugar has dissolved.
As soon as the cake comes out of the oven, place the pan on a wire rack, poke the cake all over with a wire tester or wooden skewer, and brush it with half the syrup.
Cool the cake in the pan for 10 minutes.
Loosen the sides of the cake with a spatula, and invert it onto a greased wire rack.
Poke the bottom of the cake with the wire tester, brush it with some syrup, and re-invert it onto a greased wire rack.
Brush the sides with the remaining syrup and allow the cake to cool completely.
Store the cake for 24 hours, tightly wrapped in plastic wrap (to give the syrup a chance to distribute evenly). Serve at room temperature. *Note*: Tightly wrapped, the cake will keep for 3 days at room temperature, 1 week in the refrigerator, and 2 months in the freezer.