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. Set aside for later use.
03 - Beat softened butter and granulated sugar in a large bowl until light and fluffy, approximately 3 minutes. The mixture should appear pale and increased in volume.
04 - Add eggs one at a time, mixing thoroughly after each addition. Incorporate lemon zest, lemon juice, and vanilla extract until fully combined.
05 - Gradually add dry ingredients to the wet mixture in three additions, alternating with milk. Begin and end with dry ingredients. Mix only until just combined to avoid developing gluten.
06 - Transfer batter to the prepared pan, spreading evenly and smoothing the top with a spatula to ensure uniform baking.
07 - Bake for 45-50 minutes until a toothpick inserted into the center emerges clean. The cake should be golden brown and spring back when lightly touched.
08 - Let the cake rest in the pan for 15 minutes to set, then carefully invert onto a wire rack to cool completely before glazing.
09 - Whisk powdered sugar with enough lemon juice to achieve a thick but pourable consistency. Drizzle evenly over the completely cooled cake just before serving.