Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners or grease.
Whisk flour, sugar, poppy seeds, baking powder, baking soda, and salt in a large bowl.
In another bowl, whisk butter, eggs, vanilla, buttermilk, lemon zest, and juice.
Fold wet ingredients into dry until just combined.
Divide batter into muffin cups, filling 2/3 full. Sprinkle sugar on top.
Bake 18–22 minutes. Cool slightly, then transfer to a rack.
Mix powdered sugar and lemon juice for glaze, drizzle over muffins, and sprinkle extra sugar if desired.