01 - Preheat oven to 350°F. Thoroughly grease and flour a 10 to 12-cup bundt pan.
02 - In a medium bowl, whisk together flour, poppy seeds, baking powder, baking soda, and salt. Set aside.
03 - In a large bowl, beat butter and sugar using a mixer until light and fluffy, about 3 to 4 minutes.
04 - Add eggs one at a time, beating well after each addition.
05 - Mix in lemon zest, lemon juice, and vanilla extract until combined.
06 - On low speed, alternately add the flour mixture and sour cream in three parts, starting and ending with flour mixture. Avoid overmixing.
07 - Pour batter evenly into the prepared pan and smooth surface.
08 - Bake for 45 to 50 minutes, until a toothpick inserted in center comes out clean.
09 - Let cool in the pan for 15 minutes, then invert onto a wire rack to cool completely.
10 - Whisk powdered sugar with enough lemon juice to achieve a thick, pourable glaze. Drizzle over cooled cake.