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 pale and slightly thickened, about 2 minutes.
04 - Add milk, sour cream or yogurt, melted butter, lemon zest, lemon juice, and vanilla extract to the egg mixture; whisk until fully combined.
05 - Gently fold dry ingredients into wet mixture just until no flour streaks remain, avoiding overmixing.
06 - Pour batter into prepared loaf pan and smooth the surface evenly.
07 - Bake 45 to 55 minutes until a toothpick inserted in the center comes out clean. Tent with foil if browning too quickly in last 10 minutes.
08 - Allow loaf to cool in the pan for 15 minutes. Remove from pan and cool completely on a wire rack.
09 - Whisk powdered sugar with lemon juice until smooth and drizzle over cooled loaf prior to slicing.