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, baking powder, baking soda, salt, and poppy seeds. Set aside.
03 - In a large bowl, beat butter and sugar on medium speed with an electric mixer until light and fluffy, about 3 to 4 minutes.
04 - Add eggs one at a time, beating well after each addition. Stir in lemon zest and vanilla extract.
05 - In a small bowl, mix buttermilk with fresh lemon juice.
06 - On low speed, alternately add the flour mixture and buttermilk mixture to the butter mixture in three parts, beginning and ending with the flour mixture. Mix until just combined, avoiding overmixing.
07 - Pour batter evenly into the prepared bundt pan and smooth the surface.
08 - Bake for 40 to 45 minutes, or until a toothpick inserted into the center comes out clean.
09 - Allow cake to cool in the pan for 15 minutes. Then invert onto a wire rack and cool completely.
10 - Whisk together powdered sugar, lemon juice, and lemon zest until smooth and pourable. Adjust lemon juice quantity for desired consistency.
11 - Drizzle glaze evenly over the cooled cake. Let set before slicing and serving.