01 - Preheat oven to 350°F. Grease and line a 9x5-inch loaf pan with parchment paper.
02 - Whisk together flour, poppy seeds, baking powder, baking soda, and salt in a medium bowl.
03 - In a large bowl, whisk sugar and eggs until light and slightly thickened, about 2 minutes.
04 - Add milk, sour cream (or yogurt), melted butter, lemon zest, lemon juice, and vanilla to the egg mixture and whisk until combined.
05 - Gently fold dry ingredients into the wet mixture until just combined; avoid overmixing.
06 - Pour batter into prepared pan, smooth the surface, and bake for 45–55 minutes until a toothpick inserted comes out clean. Tent with foil last 10 minutes if browning too quickly.
07 - Cool loaf in pan for 15 minutes, then transfer to a wire rack and cool completely.
08 - Whisk powdered sugar with lemon juice until smooth and drizzle over cooled loaf before slicing.