01 - Preheat oven to 350°F. Grease and flour a 10-12 cup bundt pan thoroughly to prevent sticking.
02 - In a medium bowl, whisk together flour, poppy seeds, baking powder, baking soda, and salt until well blended. Set aside.
03 - In a large bowl, beat the softened butter and granulated sugar together with an electric mixer until light and fluffy, approximately 3-5 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 mix in the flour mixture in three additions, alternating with the sour cream and milk. Begin and end with the flour mixture. Mix only until just incorporated to avoid toughening the cake.
06 - Transfer batter evenly into the prepared bundt pan. Smooth the top surface with a spatula for even baking.
07 - Bake for 45-50 minutes, or until a wooden toothpick inserted into the center emerges clean or with just a few moist crumbs.
08 - Allow the cake to cool in the pan for 15 minutes, then carefully invert onto a wire rack. Let cool completely before applying icing.
09 - Whisk together powdered sugar, lemon juice, and lemon zest until smooth and pourable. Adjust consistency with additional lemon juice if necessary.
10 - Drizzle the icing evenly over the completely cooled cake. Allow icing to set for 15-20 minutes before slicing and serving.