01 - Preheat the oven to 350°F. Grease and flour a 10- to 12-cup bundt pan thoroughly.
02 - Whisk together flour, baking powder, baking soda, salt, and poppy seeds in a medium bowl. Set aside.
03 - Using an electric mixer on medium speed, cream the butter and sugar in a large bowl until light and fluffy, approximately 3 to 4 minutes.
04 - Beat in the eggs one at a time, mixing well after each addition. Incorporate the lemon zest and vanilla extract.
05 - In a small bowl, mix the buttermilk with fresh lemon juice.
06 - On low speed, add the flour mixture and buttermilk mixture to the butter mixture alternately in three additions, starting and ending with the flour mixture. Mix just until combined to avoid overmixing.
07 - Pour the batter evenly into the prepared bundt pan and smooth the surface. Bake for 40 to 45 minutes, or until a toothpick inserted in the center comes out clean.
08 - Allow the cake to cool in the pan for 15 minutes before inverting onto a wire rack to cool completely.
09 - Whisk together powdered sugar, lemon juice, and lemon zest until smooth and pourable. Adjust lemon juice to achieve desired consistency.
10 - Drizzle the glaze evenly over the cooled cake. Let the glaze set before slicing and serving.