Prep oven and pans. Preheat oven to 350F and position oven rack in the middle of your oven. Grease and flour (or line with parchment paper) two 9x5-inch loaf pans. Or you can bake this in a 12-cup capacity bundt pan. I like to spray the pans first, then line the pans with parchment paper. And then spray one more time. If using a bundt pan, do NOT grease and flour until right before adding the batter.
Combine dry ingredients. In a large mixing bowl, whisk together the flour, baking powder, baking soda, and salt together in a bowl and set aside.
Combine sour cream mixture. In another bowl, combine the lemon zest, vanilla extract and sour cream. Set aside.
Cream butter and sugar. In a mixing bowl, with an electric mixer, cream together the butter and sugar until light and fluffy, on medium speed for about 2-3 minutes. Scrape down the bowl as needed.
Add eggs. Add the eggs one at a time, scraping down the mixing bowl well after each addition and mix until blended. Your batter may look slightly curdled at this point, but will smooth out once the flour has been added.
Alternate dry ingredients and sour cream mixture. Add the dry ingredients and sour cream mixture alternately to butter-sugar-egg mixture until uniformly incorporated- do not overmix. Start and end with the flour mixture, in 3 additions.
Add poppyseeds. Stir in the poppy seeds until evenly incorporated. Do not overmix. I like to stir these in with a spatula to ensure I do not overmix at this point.
Bake. Pour batter into prepared pan(s). Bake in a preheated 350 degrees oven for approximately 40-45 minutes (or for a bundt pan bake for 50-55 minutes). The bread is done when a toothpick or skewer inserted in the center comes out clean. A few moist crumbs is Ok! Remove pans from the oven and allow to cool on a cooling rack for 10-15 minutes. Then remove from the pans and continue to cool on a cooling rack. Allow to cool before glazing.
Make the glaze. Mix all ingredients until smooth. You can adjust the consistency of the glaze by adding more powdered sugar, or more lemon juice. When bread is cooled, divide the glaze between the two breads, and pour over top of breads and serve. For a thicker glaze use less lemon juice.