Pre-heat the oven to 350°F (175°C). Grease, flour, and the line the bottoms of three 8-inch (20-cm) cake pans with parchment paper and set aside.
Sift together the flour, baking powder, baking soda, and salt. Stir in the poppy seeds and set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium speed for 2 minutes. Meanwhile, rub the sugar and lemon zest together in a small bowl between your fingertips until fragrant. Add the sugar mixture and mix on medium-high until light and fluffy, 3 to 5 minutes. Stop the mixer and scrape down the bowl.
Turn the mixer to medium-low and add the vanilla and eggs, one at a time. Mix until combined. Stop the mixer and scrape down the bowl.
Combine the buttermilk and lemon juice together in a liquid measuring cup or bowl. Turn the mixer to low and add the flour mixture in three batches, alternating with the buttermilk mixture, beginning and ending with the flour mixture. Mix on medium for no more than 30 seconds after the last streaks of the flour mixture are combined.
Evenly distribute the batter between the prepared pans. Bake for 26 to 28 minutes or until a toothpick inserted into the center of the cakes comes out clean. Let them cool on a wire rack for 10 to 20 minutes before removing the cakes from their pans. Once cool, remove the parchment paper and discard.