Preheat your oven to 350°F (175°C). Grease and line a 9x5-inch loaf pan with parchment paper to prevent sticking.
Prepare the dry ingredients: In a bowl, whisk together 1 3/4 cups all-purpose flour, 2 tablespoons poppy seeds, 1 1/2 teaspoons baking powder, and 1/2 teaspoon salt. Set this aside.
Beat the wet ingredients: In a separate bowl, combine 1 cup granulated sugar, 3/4 cup (1 1/2 sticks) softened butter, and 1 tablespoon lemon zest. Beat on medium speed until fluffy, about 3-4 minutes.
Add the eggs: Add 3 large eggs one at a time to the mixture, ensuring each one is fully incorporated before adding the next. Then, mix in 2 tablespoons fresh lemon juice and 2 teaspoons vanilla extract until combined.
Combine wet and dry ingredients: Gradually mix in the dry ingredients along with 1/4 cup buttermilk and 1/4 cup sour cream. Stir until just combined, being careful not to overmix.
Transfer to the pan: Pour the batter into the prepared loaf pan and smooth the top with a spatula.
Bake: Place the loaf in the oven and bake for 45-50 minutes, or until a toothpick inserted into the center comes out clean. Keep an eye on it to prevent over-baking.
Cool: After baking, let the bread cool in the pan for 10 minutes. Carefully transfer it to a wire rack to cool completely.
Prepare the glaze: In a separate bowl, whisk together 1 cup powdered sugar and 1-2 tablespoons fresh lemon juice until smooth. Adjust the lemon juice according to your desired consistency.
Drizzle: Once the bread has completely cooled, drizzle the glaze over the top, allowing it to cascade down the sides.