Preheat oven to 375°F. Line a 12-cup muffin pan with paper liners or lightly grease.
In a large bowl, whisk together flour, sugar, baking powder, baking soda, salt, poppy seeds, and lemon zest.
In a separate bowl, whisk buttermilk, lemon juice, melted butter, eggs, and vanilla until smooth.
Pour wet ingredients into dry ingredients and fold gently just until no dry streaks remain. Do not overmix.
Divide batter evenly among muffin cups, filling each about 0.75 full.
Bake 16 to 18 minutes, or until tops spring back and a toothpick inserted in the center comes out clean. Cool in pan 5 minutes, then transfer to a wire rack.
For the glaze, whisk powdered sugar with lemon juice until smooth. Add a splash of milk if needed to reach a pourable consistency.
Drizzle glaze over cooled muffins and let set for 10 minutes before serving.
Notes
For extra lemon flavor, add an additional tablespoon of zest. No buttermilk? Use 1.0 cup milk mixed with 1.0 tablespoon lemon juice; let stand 5 minutes before using. Store muffins airtight at room temperature for 2 days or freeze up to 2 months.