01 - Preheat oven to 350°F. Thoroughly grease and flour a 10-12 cup bundt pan, ensuring all crevices are coated for easy release.
02 - Whisk flour, baking powder, baking soda, salt, and poppy seeds in a medium bowl until evenly distributed. Set aside.
03 - Beat butter and sugar on medium-high speed for 3 minutes until pale, light, and fluffy. Scrape down bowl as needed.
04 - Add eggs one at a time, beating fully after each addition. Incorporate lemon juice, zest, and vanilla until smooth.
05 - With mixer on low, add flour mixture in three additions, alternating with sour cream and milk. Begin and end with flour. Mix only until just combined; avoid overmixing.
06 - Transfer batter to prepared bundt pan, spreading evenly and smoothing top with spatula.
07 - Bake for 45-50 minutes until wooden skewer inserted in center comes out clean.
08 - Let cake cool in pan for 15 minutes, then carefully invert onto wire rack. Cool completely before glazing.
09 - Whisk powdered sugar, lemon juice, and zest until smooth and pourable. Add additional juice by teaspoon if too thick.
10 - Drizzle glaze over completely cooled cake, allowing it to cascade down sides. Let set for 15 minutes before slicing.