01 - Preheat oven to 350°F. Grease and flour a 10-cup Bundt pan or 9x5-inch loaf pan.
02 - Whisk together flour, poppy seeds, baking powder, baking soda, and salt in a medium bowl. Set aside.
03 - Beat butter and sugar in a large bowl until light and fluffy, approximately 3 minutes.
04 - Add eggs one at a time, mixing thoroughly after each addition. Blend in lemon zest, lemon juice, and vanilla extract.
05 - Gradually add dry ingredients to wet mixture in three additions, alternating with milk. Begin and end with dry ingredients. Mix until just combined; avoid overmixing.
06 - Transfer batter to prepared pan and level surface. Bake for 45-50 minutes until a toothpick inserted into center emerges clean.
07 - Let cake rest in pan for 15 minutes, then invert onto wire rack to cool completely.
08 - Whisk powdered sugar with sufficient lemon juice to achieve thick yet pourable consistency. Drizzle over cooled cake prior to serving.