Prepare oven and bundt cake pan: Preheat oven to 350 F degrees. Spray a bundt cake pan generously with cooking spray and set aside.
Combine dry ingredients: In a large bowl combine the flour, baking powder, salt and poppy seeds together. Set aside.
Mix wet ingredients: In the bowl of your mixer add the sugar and lemon zest. Using the whisk attachment whisk for a couple minutes until the lemon zest and sugar are well blended. Add the butter and beat until mixture is light and fluffy, about 4 minutes.
Add the buttermilk and eggs to the mixer and mix well for about 2 to 3 minutes until smooth.
Combine wet and dry: Add half the flour mixture to the mixer and mix until well incorporated. Add the remaining flour and continue mixing until well incorporated. Pour batter into the prepared baking pan.
Bake: Bake for 55 to 65 minutes or until an inserted toothpick in the center of the loaf comes out clean.
Make glaze: While the cake is baking, make the lemon glaze by whisking the powdered sugar, lemon juice and vanilla extract. This glaze needs to be quite runny, so add milk as needed, about a tbsp at a time until the desired consistency is achieved.
Wait before taking the cake out of the pan: Take the cake out of the oven and let it sit in the bundt pan for 10 minutes. Run a knife along the sides of the cake if necessary, then flip the cake over onto a cake tray. Let the cake cool for a bit for about 10 to 20 minutes.
Drizzle with lemon glaze and serve: Drizzle the cake with the lemon glaze, cut into slices and serve.