1. Preheat your oven to 350°F (180°C). Grease a 9×5-inch loaf pan with butter and lightly flour it.
2. In a medium bowl, sift together the flour, baking powder, baking soda, and salt. Set aside.
3. In a large mixing bowl, cream the softened butter and sugar together on medium speed until pale and fluffy.
4. Add the eggs one at a time, beating well after each addition. Stir in the lemon juice, lemon zest, and poppy seeds.
5. Reduce the mixer speed to low. Alternately add half of the flour mixture and half of the milk, mixing gently until just combined. Repeat with the remaining flour and milk. Optionally, fold in the sour cream.
6. Pour the batter into the prepared loaf pan. Bake for 50–60 minutes, or until a toothpick inserted in the center comes out clean.
7. Cool the loaf in the pan for about 10 minutes, then transfer to a wire rack to cool completely.
8. For the glaze, whisk together the powdered sugar, lemon juice, and milk until smooth. Drizzle or brush over the cooled loaf.