01 - Preheat oven to 350°F. Grease and flour a 12-cup bundt pan thoroughly, ensuring all crevices are coated to prevent sticking.
02 - Whisk together flour, poppy seeds, baking powder, baking soda, and salt in a medium bowl until uniformly blended.
03 - Beat butter and sugar with electric mixer on medium-high speed until light and fluffy, approximately 3-4 minutes.
04 - Add eggs one at a time, mixing thoroughly after each addition. Blend in lemon zest, lemon juice, and vanilla extract until smooth.
05 - Add dry ingredients in three portions, alternating with buttermilk, beginning and ending with dry ingredients. Mix only until just combined to avoid developing gluten.
06 - Transfer batter evenly into prepared bundt pan, smoothing surface with spatula to ensure even baking.
07 - Bake for 38-42 minutes until toothpick inserted in center emerges clean or with few moist crumbs.
08 - Let cake rest in pan for 15 minutes. Invert onto wire rack and cool completely before glazing.
09 - Whisk powdered sugar and lemon juice until smooth and pourable. Adjust consistency with additional juice if needed.
10 - Drizzle glaze evenly over cooled cake. Allow glaze to set for 15-20 minutes before slicing and serving.