Preheat the oven to 325 degrees F. Generously and thoroughly grease and flour a 12-cup Bundt pan (I grease, sugar, and flour the pan – see Notes for instructions).
Whisk together the flour, poppy seeds, baking powder, baking soda, and salt in a medium bowl. Set the mixture aside.
Whisk together the buttermilk, lemon juice, lemon zest, and vanilla in a liquid measuring cup or medium mixing bowl, then set aside.
Add the butter and sugar to a stand mixer fitted with the paddle attachment and beat on medium-high speed for at least 3-4 minutes (no less!), until light and fluffy, scraping down sides occasionally. Reduce the speed to medium and add the eggs, one at a time, beating just until the yellow disappears after each egg.
Reduce the speed to low and gradually add the flour mixture to the butter mixture in three parts, alternating with the buttermilk mixture. Beat until just combined, and then give the batter a final stir by hand to ensure all the flour is mixed in.
Transfer the batter to the prepared Bundt pan and tap the pan on the counter a few times to remove any air bubbles. Bake at 325 degrees F for 45- 55 minutes until a wooden pick inserted near the center of the cake comes out with a few crumbs, 45- 55 minutes.
Let the cake cool in the pan on a wire rack for 10 minutes then invert it onto the rack to cool completely before adding the glaze.