01 - Preheat oven to 350°F. Grease and flour a 12-cup bundt pan thoroughly, ensuring all crevices are coated.
02 - Whisk together flour, poppy seeds, baking powder, baking soda, and salt in a medium bowl. Set aside.
03 - Beat butter and sugar with an electric mixer until light and fluffy, about 3–4 minutes.
04 - Add eggs one at a time, mixing well after each addition. Stir in lemon zest, lemon juice, and vanilla extract.
05 - Add dry ingredients in three parts, alternating with buttermilk, beginning and ending with dry ingredients. Mix just until combined; avoid overmixing.
06 - Spoon batter evenly into prepared bundt pan and smooth the top with a spatula.
07 - Bake 38–42 minutes until a toothpick inserted into the center emerges clean.
08 - Cool in pan 15 minutes, then invert onto a wire rack to cool completely.
09 - Whisk powdered sugar and lemon juice until smooth and pourable. Drizzle over cooled cake and let set before slicing.