01 - Preheat oven to 350°F. Grease and flour a 10-cup Bundt pan or 9x5-inch loaf pan thoroughly to prevent sticking.
02 - Whisk together flour, poppy seeds, baking powder, baking soda, and salt in a medium bowl until evenly distributed.
03 - Beat softened butter and granulated sugar in a large bowl until light and fluffy, approximately 3 minutes.
04 - Add eggs one at a time, beating thoroughly after each addition. Mix in lemon zest, lemon juice, and vanilla extract until combined.
05 - Gradually add dry ingredients to the 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 smooth the surface. Bake for 45-50 minutes until a toothpick inserted into the center comes out clean.
07 - Let cake cool in the pan for 15 minutes, then invert onto a wire rack to cool completely.
08 - Whisk powdered sugar with sufficient lemon juice to achieve a thick but pourable consistency. Drizzle glaze over the completely cooled cake before serving.